likely from the adjective. Related: Fitted; fitting.
"paroxysm, sudden attack" (as of anger), 1540s, probably via M.E. sense of "painful, exciting experience," from O.E. fitt "conflict, struggle," of uncertain origin, with no clear cognates outside English. Phrase by fits and starts first attested 1610s.
"suited to the circumstances, proper," mid-15c., of unknown origin, perhaps from M.E. noun fit "an adversary of equal power" (mid-13c.), which is perhaps connected to fit
(n.1). The verb meaning "to be the right shape" is first attested 1580s. Survival of the fittest (1867) coined by H. Spencer.