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tic (n.): twitching of a facial muscle, 1822, often a shortening of tic douloureux "severe facial neuralgia," literally "painful twitch" (1798), from French tic "a twitching disease of horses" (17c.), of unknown origin. Klein suggests an imitative origin; Diez compare it to Italian ticchio "whim, caprice, ridiculous habit," itself of unknown origin.
tic douloureux (n.): 1798, French, literally "painful twitching;" see tic.
twitch (v.): late 12c., to-twic-chen "pull apart with a quick jerk," related to Old English twiccian "to pluck, gather, catch hold of," from Proto-Germanic *twikjon- (cognates: Low German twicken, Dutch twikken, Old High German gizwickan, German zwicken "to pinch, tweak"). Related: Twitched; twitching.
atrial fibrillation: "Irregular, uncoordinated twitching or quivering (as opposed to regular contraction) of the atria of the heart, typically associated with irregular and often rapid ventricular contraction", Early 20th cent.; earliest use found in Journal of the American Medical Association.