1550s, in literal sense "mix or mingle things so as to render the elements indistinguishable;" attested from mid-18c. in active, figurative sense of "discomfit in mind or feeling;" not in general use until 19c., taking over senses formerly belonging to confound, dumbfound, flabbergast etc. The past participle confused, q.v. is attested much earlier (serving as an alternative past tense to confound), and the verb here might be a back-formation from it. Related: Confusing.