late 14c., "appearance, guise; quality or state of being similar; a comparison; person or thing that resembles another," from Old French similitude "similarity, relationship, comparison" (13c.) and directly from Latin similitudinem (nominative similitudo) "likeness, resemblance," from similis "like, resembling, of the same kind" (see similar).
Middle English kitthe "people, race, kinsmen, family," also "homeland, native region; kinship, relationship; knowledge, news; propriety, custom," from Old English cyðð "kinship, relationship; kinsfolk, fellow-countrymen, neighbors; native country, home; knowledge, acquaintance, familiarity," from cuð "known," past participle of cunnan "to know" (see can (v.)), from PIE root *gno- "to know."
The alliterative phrase kith and kin (late 14c.) originally meant "country and kinsfolk" and is almost the word's only survival in Modern English. Some cognates have evolved different senses, such as Dutch kunde "skill, competence," German Kunde "knowledge, news, tidings."
mid-14c., "nobility of birth, gentle birth," from Old French gentilité (14c.), from Latin gentilitatem (nominative gentilitas) "relationship in the same family or clan," from gentilis "of the same family or clan" (see gentle; also compare gentry). From 1640s as "social superiority." Meaning "state of being gentile" is rare.
1540s, "pertaining to marriage, nuptial," also "pertaining to the relationship of husband and wife," from French conjugal (13c.), from Latin coniugalis "relating to marriage," from coniunx (genitive coniugis) "spouse," which is related to coniugare "to join together," from assimilated form of com "with, together" (see con-) + iugare "to join," from iugum "yoke" (from PIE root *yeug- "to join"). Related: Conjugacy; conjugality.
1630s, "action based on natural instincts," from natural (adj.) + -ism. In philosophy, as a view of the world and humanity's relationship to it involving natural forces only (and excluding spiritualism and superstition), from 1750. As a tendency in art and literature, "conformity to nature or reality, but without slavish fidelity to it," from 1850.
1520s, "process of becoming, or state of being, a son," from French filiation, from Medieval Latin filiationem (nominative filiatio), noun of action from past-participle stem of filiare "to have a child," from Latin filius/filia "son/daughter" (see filial). As "relationship of a son or daughter to a parent" (correlative of paternity) from 1794.
early 15c., relatif, "having reference (to something), relating, depending upon," from Old French relatif and directly from Late Latin relativus "having reference or relation," from Latin relatus, used as past participle of referre "bring back, bear back" (see refer), from re- "back, again" + lātus "borne, carried" (see oblate (n.)).
Meaning "having mutual relationship, connected with each other" is from 1590s; that of "arising from or determined by relationship to something else" is from 1610s; that of "having or standing in a relation to something else" is from 1650s; that of "not absolute or existing by itself" is by 1704. In grammar, "referring to an antecedent," from 1520s.
1894, "anyone of a relationship not natural," abstracted from father-in-law, etc.
The position of the 'in-laws' (a happy phrase which is attributed ... to her Majesty, than whom no one can be better acquainted with the article) is often not very apt to promote happiness. [Blackwood's Magazine, 1894]
The earliest recorded use of the formation is in brother-in-law (13c.); the law is Canon Law, which defines degrees of relationship within which marriage is prohibited. Thus the word originally had a more narrow application; its general extension to more distant relatives of one's spouse is, according to OED "recent colloquial or journalistic phraseology." Middle English inlaue (13c.) meant "one within or restored to the protection and benefit of the law" (opposite of an outlaw), from a verb inlauen, from Old English inlagian "reverse sentence of outlawry."