mid-15c., "full of pride," from Latin fastidiosus "disdainful, squeamish, exacting," from fastidium "loathing, squeamishness; dislike, aversion; excessive nicety," which is of uncertain origin; perhaps from *fastu-taidiom, a compound of fastus "contempt, arrogance, pride," and taedium "aversion, disgust." Fastus is possibly from PIE *bhars- (1) "projection, bristle, point," on the notion of "prickliness" (Watkins) or "a semantic shift from 'top' to 'haughtiness' which is conceivable, but the u-stem is not attested independently" [de Vaan], who adds that "fastidium would be a tautology." Early use in English was both in passive and active senses. Meaning "squeamish, over-nice" in English emerged 1610s. Related: Fastidiously; fastidiousness.
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Harper, D. (n.d.). Etymology of fashious. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/fashious