1680s, "process of fitting," from fit (v.). From 1823 as "the fitting of one thing to another;" 1831 as "the way something fits."
"paroxysm, sudden attack" (as of anger), 1540s, probably via Middle English sense of "painful, exciting experience" (early 14c.), from Old English fitt "conflict, struggle," which is of uncertain origin, with no clear cognates outside English. Perhaps ultimately cognate with fit (adj.) on notion of "to meet." Meaning "sudden impulse toward activity or effort" is from 1580s. Phrase by fits and starts first attested 1610s (by fits is from 1580s).
"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). In athletics, "in condition, properly trained for action," from 1869. Related: Fitter; fittest. Survival of the fittest (1867) coined by H. Spencer.
c. 1400, "to marshal or deploy (troops);" early 15c. as "be fitting or proper, be suitable," from fit (adj.) and perhaps in part from Scandinavian (compare Old Norse fitja "knit"). From 1580s as "be the right shape." Transitive sense of "provide with what is suitable" is from 1590s; that of "make fit or suitable, bring into corresponding form or condition" is from c. 1600. Related: Fitted; fitting. Fitted sheets is attested from 1948.