c. 1300, "to condemn, curse," also "to destroy utterly;" from Anglo-French confoundre, Old French confondre (12c.) "crush, ruin, disgrace, throw into disorder," from Latin confundere "to confuse, jumble together, bring into disorder," especially of the mind or senses, "disconcert, perplex," properly "to pour, mingle, or mix together," from assimilated form of com "together" (see con-) + fundere "to pour" (from nasalized form of PIE root *gheu- "to pour").
From mid-14c. as "to put to shame, disgrace." The figurative sense of "confuse the mind, perplex" emerged in Latin, passed into French and thence to English by late 14c. The Latin past participle confusus, meanwhile, became confused (q.v.). The meaning "treat or regard erroneously as identical" is from 1580s.