dumbfound (v.)

Old English dumb, of persons, "mute, silent, refraining from speaking or unable to speak," from Proto-Germanic *dumbaz "dumb, dull," which is perhaps from PIE *dheubh- "confusion, stupefaction, dizziness," from root *dheu- (1) "dust, mist, vapor, smoke," also expressing related notions of "defective perception or wits." The -b has probably been silent since 13c. Related: Dumbly; dumber; dumbest. Of animals, "lacking in speech," hence "without intellect" (c. 1200).

The fork in meaning probably comes via the notion of  "not responding through ignorance or incomprehension." The Old English, Old Saxon (dumb), Gothic (dumbs), and Old Norse (dumbr) forms of the word meant only "mute, speechless;" in Old High German (thumb) it meant both this and "stupid," and in Modern German this latter became the only sense (the sense of "mute, speechless" being expressed by stumm). Meaning "foolish, ignorant" was occasional in Middle English, but the modern use in this sense (since 1823) seems to be from influence of German dumm, especially in Pennsylvania German.

dumb-cake ..., n. A cake made in silence on St Mark's Eve, with numerous ceremonies, by maids, to discover their future husbands. [Century Dictionary]

Applied to silent contrivances, hence dumb-waiter. Dumb ox "stupid man" is by 1756; dumb-bunny "stupid person" is college slang from 1922; dumb blonde "woman seen as incapable of comprehending anything complicated" is by 1936.

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.

"stricken dumb, confused, perplexed," 1680s, past-participle adjective from dumbfound.