Definition: elegant, fashionable
The notion that posh is an acronym for “port out, starboard home” is one of the more enduring myths of English etymology, resistant to common sense, probability, and copious amounts of research. This phrase is not the origin of posh.
"Port out, starboard home” is thought to refer to the desirability of certain cabin accommodations on ships traveling between Britain and India; on the way out the port side of the ship would be preferred, as it received less sun, as was the case with the starboard side on the way back. Despite what you may have heard, tickets (or at least all the surviving ones from that time) were not stamped with the letters POSH.
We do have a good deal of evidence of the word’s use from the early 20th century, and one thing
about it that is striking is that most of it comes from the British military. These uses were generally by lower-ranking troops, not the sort who would be picking which cabin they wanted for a liner voyage to India. The actual origin of the word is unknown.
The Regiment was inspected by the G.O.C. Commanding 2nd Army. The parade looked very “posh” and everything appeared to pass off satisfactorily.
—The East Kent Yeoman, 1915
Deakins of old ‘D’ Company, is now a Battalion H. Q. Cook—a very ‘posh’ job.
—The Londoner: The Journal of the 1/25th Battalion, 1918
And there is a compensation for the hard work. It consists of our “posh” clothes. When you see a swanky looking lad in an officer’s uniform wearing a white band round his cap, you will know that he is one of us.
—The Times of India (New Delhi, India), 2 Nov. 1917
Sir,—Your contributor “O. S. P.,” in his article “War Words and Phrases,” omits one word in common use among soldiers at the front. It is the word “posh,” which appears to have more than one meaning, as in the case of “strafe.” I have heard a good meal described as “very posh,” and it is also use as a substitute for “swank” on occasion.
—J. Wallace Black, (letter) Daily Mail (London, England), 3 Nov. 1916