Old English fyst "fist, clenched hand," from West Germanic *fusti- (source also of Old Saxon fust, Old High German fust, Old Frisian fest, Middle Dutch vuust, Dutch vuist, German Faust), from Proto-Germanic *funhstiz, probably ultimately from a PIE "hand" word that is ultimately cognate with the root *penkwe- "five" (compare Old Church Slavonic pesti, Russian piasti "fist"), in reference to the five fingers.
Meaning "a blow with the fist" is from 1767. Fist-fight "duel with the fists" is from c. 1600. As a verb, Old English had fystlian "to strike with the fist."
