An antiphrasis (from the, antí, "opposite" and, phrásis, "diction")'s a figure of speech that's a word used to mean the opposite of its usual sense, especially ironically. Examples: "The past's strongest in its resurrection; but she'll have neither." (succumbing to nostalgia; from "Three-Fold Flowers" by Andrea Duerme)… (More on Antiphrasis)