fit (n.1)
1823, "the fitting of one thing to another," later (1831) "the way something fits." Originally "an adversary of equal power" (mid-13c.), obscure, possibly from Old English fitt "a conflict, a struggle" (see fit (n.2)).
fit (v.)
"be suitable," probably from early 15c.; "to be the right shape," 1580s, from fit (adj.). Related: Fitted; fitting. Fitted sheets is attested from 1963.
fit (n.2)
"paroxysm, sudden attack" (as of anger), 1540s, probably via Middle English sense of "painful, exciting experience" (early 14c.), from Old English fitt "conflict, struggle," of uncertain origin, with no clear cognates outside English. Perhaps ultimately cognate with fit (n.1) on notion of "to meet." Phrase by fits and starts first attested 1610s.
fit (adj.)
"suited to the circumstances, proper," mid-15c., of unknown origin, perhaps from Middle English noun fit "an adversary of equal power" (mid-13c.), which is perhaps connected to fit (n.1). Related: Fitter; fittest. Survival of the fittest (1867) coined by H. Spencer.
fit (n.3)
part of a poem, Old English fitt, of unknown origin.