the verb 'twitch' entered middle english around the 12th century as 'twicchen', meaning 'to pluck or pull sharply'. it is believed to be of germanic origin, likely from an old english root related to 'twiccian', meaning 'to pluck', which is connected to the middle low german 'twikken'. the core semantic idea is a quick, pulling motion. this evolved to describe the quick, involuntary pulling or jerking of a muscle or body part. the '-ing' suffix is a native english grammatical element used to form nouns describing the action or state (as in 'a twitching of the eye') or adjectives describing something performing that action (as in 'a twitching muscle').

the word 'twitching' is a simplex form derived from the verb 'twitch'. it is not productively composed of separate, meaningful morphemes in modern english. the '-ing' suffix is a modern english inflectional suffix used to form the present participle/gerund or an adjective, but the core 'twitch' is an unsegmentable unit.