by 1764, from kin + -ship. Relationship covers the same sense but is a hybrid.
Entries linking to kinship
kin n.
c. 1200, from Old English cynn "family; race; kind, sort, rank; nature" (also "gender, sex," a sense obsolete since Middle English), from Proto-Germanic *kunja- "family" (source also of Old Frisian kenn, Old Saxon kunni "kin, kind, race, tribe," Old Norse kyn, Old High German chunni "kin, race;" Danish kjön, Swedish kön, Middle Dutch, Dutch kunne "sex, gender;" Gothic kuni "family, race," Old Norse kundr "son," German Kind "child"), from PIE root *gene- "give birth, beget," with derivatives referring to procreation and familial and tribal groups.
In the Teutonic word, as in Latin genus and Greek [genos], three main senses appear, (1) race or stock, (2) or kind, (3) gender or sex .... [OED]
Related to both words kind and to child. From 1590s as an adjective, from the noun and as a shortening of akin. Legal next of kin (1540s) does not include the widow, "she being specifically provided for by the law as widow" [Century Dictionary], and must be a blood relation of the deceased.
-ship
word-forming element meaning "quality, condition; act, power, skill; office, position; relation between," Middle English -schipe, from Old English -sciepe, Anglian -scip "state, condition of being," from Proto-Germanic *-skepi- (cognates: Old Norse -skapr, Danish -skab, Old Frisian -skip, Dutch -schap, German -schaft), from *skap- "to create, ordain, appoint," from PIE root *(s)kep-, forming words meaning "to cut, scrape, hack" (see shape (v.)). It often forms abstracts to go with corresponding concretes (friend/friendship, etc.).
relationship n.
1640s, "sense or state of being related" by kindred, affinity, or other alliance, from relation + -ship. Specifically of romantic or sexual intimacy by 1944.