"having characteristics in common," 1610s (earlier similary, 1560s), from French similaire, from a Medieval Latin extended form of Latin similis "like, resembling, of the same kind," from Old Latin semol "together" (from PIE root *sem- (1) "one; as one, together with"). The noun meaning "that which is similar" is from 1650s. Related: Similarly.
1590s, from Latin simulacrum "likeness, image, form, representation, portrait," a dissimilation of *simulaclom, from simulare "to make like, imitate, copy, represent," from stem of similis "like, resembling, of the same kind" (see similar). The word was borrowed earlier as semulacre (late 14c.), via Old French simulacre.