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early 14c., "discomfited, routed, defeated" (of groups), serving at first as an alternative past participle of confound, as Latin confusus was the past participle of confundere "to pour together, mix, mingle; to join together;" hence, figuratively, "to throw into disorder; to trouble, disturb, upset." The Latin past participle also was used as an adjective, with reference to mental states, "troubled, embarrassed," and this passed into Old French as confus "dejected, downcast, undone, defeated, discomfited in mind or feeling," which passed to Middle English as confus (14c.; e.g. Chaucer: "I am so confus, that I may not seye"), which then was assimilated to the English past participle pattern by addition of -ed. Of individuals, "discomfited in mind, perplexed," from mid-14c.; of ideas, speech, thought, etc., from 1610s. By mid-16c., the word seems to have been felt as a pure adj., and it evolved a back-formed verb in confuse. Few English etymologies are more confused.
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.