cousinyoudaoicibaDictYouDict[cousin 词源字典]
cousin: [13] The word cousin is etymologically related to sister. It comes via Old French cosin from Latin consobrīnus, which meant literally ‘child of one’s mother’s sister’ – that is, ‘cousin on one’s mother’s side’ (consobrīnus was a compound noun formed from the prefix com- ‘together’ and sobrīnus ‘maternal cousin’, a derivative of soror ‘sister’ and relative of English sister).

By the time it entered English, it had already broadened out in meaning to cover paternal as well as maternal cousins, and indeed in the Middle Ages it was applied more generally still to any relative other than one’s parents or brother and sister (probably through association with Latin consanguineus ‘blood relative’).
=> sister[cousin etymology, cousin origin, 英语词源]
cousin (n.)youdaoicibaDictYouDict
mid-12c., from Old French cosin (12c., Modern French cousin) "nephew, kinsman, cousin," from Latin consobrinus "cousin," originally "mother's sister's son," from com- "together" (see com-) + sobrinus (earlier *sosrinos) "cousin on mother's side," from soror (genitive sororis) "sister."

Italian cugino, Danish kusine, Polish kuzyn also are from French. German vetter is from Old High German fetiro "uncle," perhaps on the notion of "child of uncle." Words for cousin tend to drift to "nephew" on the notion of "father's nephew."

Many IE languages (including Irish, Sanskrit, Slavic, and some of the Germanic tongues) have or had separate words for some or all of the eight possible "cousin" relationships, such as Latin, which along with consobrinus had consobrina "mother's sister's daughter," patruelis "father's brother's son," atruelis "mother's brother's son," amitinus "father's sister's son," etc. Old English distinguished fæderan sunu "father's brother's son," modrigan sunu "mother's sister's son," etc.

Used familiarly as a term of address since early 15c., especially in Cornwall. Phrase kissing cousin is Southern U.S. expression, 1940s, apparently denoting "those close enough to be kissed in salutation;" Kentish cousin (1796) is an old British term for "distant relative."