c.1300, "relation by marriage" (as opposed to consanguinity), from Old French afinité (12c.), from Latin affinitatem (nominative affinitas) "neighborhood, relationship by marriage," noun of state from affinis "adjacent," also "kin by marriage," literally "bordering on," from ad- "to" (see ad-) + finis "a border, a boundary" (see finish, v.). Used figuratively since c.1600 of structural relationships in chemistry, philology, etc. Meaning "natural attraction" (as though by family) is from 1610s.