gratuitous

Like gratitude, grace, and congratulate, gratuitous is a descendant of the Latin word gratus, which means "pleasing" or "grateful." When gratuitous was first used in the middle of the 17th century, it meant "free" or "given without return benefit or compensation." The extended meaning "done without good reason" or "unwarranted" came about just a few decades later, perhaps from the belief held by some people that one should not give something without getting something in return. Today, that extended meaning is the more common sense, often used in such phrases as "a gratuitous insult" or to describe elements of a story that are not relevant to the plot.