"make different, cause to be unlike," 1821, on model of assimilate, from dis- + Latin similis "like, resembling, of the same kind," from Old Latin semol "together" (from PIE root *sem- (1) "one; as one, together with"). In linguistics, of sounds, "to become unlike, to diverge," by 1860. Related: Dissimilated; dissimilating.
c. 1600, "unlike in kind, essentially different, having no common ground," from Latin disparatus, past participle of disparare "divide, separate," from dis- "apart" (see dis-) + parare "get ready, prepare" (from PIE root *pere- (1) "to produce, procure").
The meaning seems to have been influenced in Latin by dispar "unequal, unlike" (from apparently unrelated Latin par "equal, equal-sized, well-matched"). Related: Disparately; disparateness. As a noun, "one of two or more things or characters so unlike that they cannot be compared with each other," 1580s.
c. 1400, "state or quality of being contrary, extreme opposition," from Old French contrarieté, from Late Latin contrarietatem (nominative contrarietas) "opposition," noun of quality from contrarius "opposite, opposed; contrary, reverse," from contra "against" (see contra (prep., adv.)). Meaning "something contrary to or extremely unlike another" is from mid-15c.