capyoudaoicibaDictYouDict[cap 词源字典]
cap: [OE] Old English cæppa came from late Latin cappa ‘hood’, source also of English cape ‘cloak’. The late Latin word may well have come from Latin caput ‘head’, its underlying meaning thus being ‘head covering’.
=> cappuccino, chapel, chaperone, képi[cap etymology, cap origin, 英语词源]
cap (n.)youdaoicibaDictYouDict
late Old English cæppe "hood, head-covering, cape," from Late Latin cappa "a cape, hooded cloak" (source of Spanish capa, Old North French cape, French chape), possibly a shortened from capitulare "headdress," from Latin caput "head" (see head (n.)).

Meaning "women's head covering" is early 13c. in English; extended to men late 14c. Figurative thinking cap is from 1839 (considering cap is 1650s). Of cap-like coverings on the ends of anything (such as hub-cap) from mid-15c. Meaning "contraceptive device" is first recorded 1916. That of "cap-shaped piece of copper lined with gunpowder and used to ignite a firearm" is c. 1826; extended to paper version used in toy pistols, 1872 (cap-pistol is from 1879).

The Late Latin word apparently originally meant "a woman's head-covering," but the sense was transferred to "hood of a cloak," then to "cloak" itself, though the various senses co-existed. Old English took in two forms of the Late Latin word, one meaning "head-covering," the other "ecclesiastical dress" (see cape (n.1)). In most Romance languages, a diminutive of Late Latin cappa has become the usual word for "head-covering" (such as French chapeau).
cap (v.)youdaoicibaDictYouDict
c. 1400, "to put a cap on," from cap (n.). Meaning "cover as with s cap" is from c. 1600. Figurative sense of "go one better" is from 1580s. Related: Capped; capping.