1610s, as a term in logic, from Medieval Latin identicus "the same," from Late Latin identitas "identity, sameness," ultimately from comb. form of Latin idem "the same" (from id "it, that one;" see id) + demonstrative suffix -dem. General sense of "being the same or very similar" is from 1630s. Replaced Middle English idemptical (late 15c.), from Medieval Latin idemptitas "identity," from Latin idem. Related: Identically.