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.

n.3

part of a poem, Old English fitt, of unknown origin.

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)).

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.

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.

Furious, enraged, as in I've been waiting for two hours and am fit to be tied. This expression implies anger so extreme that it requires physical restraint. [Late 1800s]

In addition to the idioms beginning with fit

• fit as a fiddle
• fit in
• fit like a glove
• fit out
• fits and starts, by
• fit to be tied
• fit to kill

also see:

• give someone fits
• have a fit
• if the shoe fits
• see fit to
• survival of the fittest