Introvert
noun : one whose personality is characterized by introversion especially : a reserved or shy person who enjoys spending time alone
We're starting with introvert because it's the most basic of the set. We all know, of course, that basic doesn't mean boring. There are numerous interesting facts about introvert, such as its use as verb, which predates its noun use. Since the mid-17th century, introvert has meant "to turn inward or in upon itself," with more specific meanings of "to concentrate or direct upon oneself" and "to produce psychological introversion in" developing over the centuries.
The adjective introverted is also older than the noun. It's been used since at least 1683, mostly with the "possessing a reserved or shy nature typically with an inclination to solitude" meaning that's contrasted with extroverted.
Meanwhile, the noun introvert has an obscure and technical use meaning "something that can be introverted"—it's applied to such remarkable appendages as the eyestalks of certain snails and the retractile proboscis of a sipunculid worm. That use dates to the late 19th century, while the one you label yourself after taking a personality test dates to the early 20th.
Aloof
adjective : removed or distant either physically or emotionally
If anyone lobs this word at you, dear introvert, feel free to stare off, as if you are looking toward a distant horizon, the promise of land after weeks at sea a tiny flame burning in your heart.
While we most often encounter aloof as an adjective, its longer history is as an adverb used with the meaning "at a distance." The adverbial use is the one American writer Herman Melville used most, typically in the collocation "hold aloof":
… I have mentioned that I used to hold myself somewhat aloof from the mass of seamen on board the Neversink …
— Herman Melville, White-Jacket, 1850
It is no surprise that aloof was in Melville's working vocabulary: it originates in nautical use. The adverb was formerly used to mean "to windward"—that is, toward the direction from which the wind is blowing. Loof is a variant of luff, which refers to the act of sailing a ship nearer the wind as well as the forward edge of a fore-and-aft sail.
Shrinking violet
noun : a bashful or retiring person
Ah, the shrinking violet. A person described thusly is of course not getting smaller; the word shrink may typically have to do with getting smaller, but it is also used to mean "to recoil instinctively" and "to hold oneself back." As for the violet bit, that word has some use with the meaning of "an overly fastidious, modest, or retiring person," as Merriam-Webster's Unabridged Dictionary reports.
Despite the human applications of each element of this compound term, shrinking violet was used literally before its figurative use developed in the late 19th century:
… while, unconscious as the daughter of Ceres, gathering flowers when the Hell King drew near, of the change that awaited her and the grim presence that approached on her fate, Helen bends still over the bank odorous with shrinking violets … — Edward Bulwer-Lytton, Lucretia, 1846
These are not a species of the Viola genus; Bulwer-Lytton was likely anthropomorphically suggesting the flowers were huddling or cowering on the bank.
MORE TO EXPLORE: Does 'Bashful' Mean "Full of Bash"?
Wallflower
noun 1 : a person who from shyness or unpopularity remains on the sidelines of a social activity (such as a dance) 2 : a shy or reserved person
In the 16th century the only wallflower referred to was an actual flower. Wallflower is the common name of a genus (technically genus Erysimum) of about 180 species of plants in the mustard family (that'd be Brassicaceae, for you botany nerds). The name comes from the fact that wallflowers often grow from chinks in walls.
The meanings of wallflower that put the word in this list date to the early 19th century, when it was often used to describe a person standing along the wall at a social function:
But such simple and sauntering stories are like Scotch reels, which have no natural ending, save the fatigue of those engaged. So I may as well cut short my mazy dance and resume at once my proper position as a 'wall flower,' with an unceremonious adieu to the kind and courteous reader. — Caroline Matilda Kirkland, A New Home—Who'll Follow?, 1839
Unclubbable
adjective : having or showing a disinclination for social activity : unsociable
Many of the unclubbable among us don't much care for clubs. We're of course not talking about the golfing implements or the suit of cards; we're talking about the kind of club 18th century lexicographer Samuel Johnson established with the artist Joshua Reynolds in 1764. "The Club," or as it was later called "The Literary Club," provided the company Johnson needed to stave off his loneliness. It also likely provided the fertile soil in which the word unclubbable could take root. The word was apparently coined by Johnson to describe a friend who perhaps had better places to be than the Club; the earliest known example of it in print is from the diary of Fanny Burney, where she quotes Johnson using the word to describe the man. (We should come clean here though: Johnson's unclubbable friend isn't the sort we'd aspire to be—his unclubbability was apparently related to his lack of desire to pay his share of the tab.) Though Johnson would surely have disagreed, we know that being unclubbable can be considered a virtue.
Buttoned-up
adjective : coldly reserved or standoffish
The word buttoned-up has described those who like to keep a cool distance from others since at least the 1760s. While in modern use the term's emphasis is typically on the restraint one so described exhibits, Charles Dickens twice applied the term to one of his most despicable characters, Mr. Vholes in Bleak House, published in 1853:
Mr. Vholes, after glancing at the official cat who is patiently watching a mouse's hole, fixes his charmed gaze again on his young client and proceeds in his buttoned-up, half-audible voice as if there were an unclean spirit in him that will neither come out nor speak out…
As he gave me that slowly devouring look of his … he gave one gasp as if he had swallowed the last morsel of his client, and his black buttoned-up unwholesome figure glided away to the low door at the end of the Hall.
The adjective's origin is in the verb phrase button up, which in its earliest 17th century figurative use was about not saying anything.
Cool
adjective : lacking ardor or friendliness
The polysemous cool may be overused, but when introverts want to describe themselves, its broad application may be part of its charm. Surely ardor—the relevant sense is defined in this dictionary as "an often restless or transitory warmth of feeling"—is not without negative effects, and friendliness to all can hardly be demanded of all. Jane Austen recognized that the introvert's cool could be useful:
She meant to avoid any such alteration of manners as might provoke a remonstrance on his side. It was a great object to her to escape all enquiry or eclat; but it was her intention to be as decidedly cool to him as might be compatible with their relationship; and to retrace, as quietly as she could, the few steps of unnecessary intimacy she had been gradually led along. She was accordingly more guarded, and more cool, than she had been the night before.
— Jane Austen, Persuasion, 1817
Taciturn
adjective : temperamentally disinclined to talk
Taciturn may not pop up in daily conversations much, but if you manage to include it in your expressive vocabulary you'll be in good company: it's a term that appears in the works of some of the English language's most esteemed dead writers, among them Austen, Brontë (both Charlotte and Emily), Conrad, Melville, Joyce, and Wharton. Living writers use it too.
I thought, if I had caused the cloud, it was my duty to make an effort to dispel it. They could not every day sit so grim and taciturn; and it was impossible, however ill-tempered they might be, that the universal scowl they wore was their everyday countenance.
— Emily Brontë, Wuthering Heights, 1847
The word is ultimately from Latin tacitus, meaning "silent"—the same source as the word tacit, meaning "expressed or carried on without words or speech" and "implied or indicated (as by an act or by silence) but not actually expressed."