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."
Proto-Indo-European root meaning "five."
It forms all or part of: cinquain; cinque; cinquecento; cinquefoil; fifteen; fifth; fifty; fin (n.) "five-dollar bill;" finger; fist; five; foist; keno; parcheesi; penta-; pentacle; pentad; Pentateuch; Pentecost; pentagon; pentagram; pentameter; pentathlon; Pentothal; Pompeii; Punjab; punch (n.2) "type of mixed drink;" quinary; quincunx; quinella; quinque-; quinquennial; quint; quintain; quintet; quintile; quintessence; quintillion; quintuple.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit panca, Greek pente, Latin quinque, Old Church Slavonic pęti, Lithuanian penki, Old Welsh pimp, Old English fif, Dutch vijf, Old High German funf.
"long, narrow ulcer," late 14c., from Latin fistula "a pipe; ulcer," which is of uncertain origin. Related: Fistular; fistulous (Latin fistulosus "full of holes; tubular").
No certain etymology. The best comparison seems to be with festuca "stalk, straw" and maybe ferula "giant fennel" (if from *fesula): the forms of a "pipe" and a "stalk" are similar. The vacillation between fest- and fist- occurs within festuca itself, and might be dialectal, or allophonic within Latin. [de Vaan]