Merriam-Webster's Collegiate® Dictionary, 11th Edition
TWITCH
I. verb
Etymology: Middle English twicchen; akin to Old English twiccian to pluck, Old High German gizwickan to pinch
Date: 14th century
transitive verb to move or pull with a sudden motion ; jerk , intransitive verb pull , pluck , to move jerkily ; quiver , to undergo a brief spasmodic muscular contraction , twitcher noun
II. noun
Date: 1523
an act of twitching, a physical or mental pang , a loop of rope or strap that is tightened over a horse's lip as a restraining device,
4. a brief spasmodic contraction of the muscle fibers, a slight jerk of a body part, twitchily adverb twitchy adjective
III. noun
Etymology: alteration of quitch
Date: 1595
quack grass