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."
Middle English -ik, -ick, word-forming element making adjectives, "having to do with, having the nature of, being, made of, caused by, similar to," from French -ique and directly from Latin -icus or from cognate Greek -ikos "in the manner of; pertaining to." From PIE adjective suffix *-(i)ko, which also yielded Slavic -isku, adjectival suffix indicating origin, the source of the -sky (Russian -skii) in many surnames. In chemistry, indicating a higher valence than names in -ous (first in benzoic, 1791).
In Middle English and after often spelled -ick, -ike, -ique. Variant forms in -ick (critick, ethick) were common in early Modern English and survived in English dictionaries into early 19c. This spelling was supported by Johnson but opposed by Webster, who prevailed.