Word Origin & History
O.E. dumb "silent, unable to speak," from PIE *dheubh- "confusion, stupefaction, dizziness." O.E., Goth. (thumb) and O.N. (dumbr) forms meant only "mute, speechless;" in O.H.G. (thumb) it meant both this and "stupid," and in Mod.Ger. this latter became the only sense. Meaning "foolish, ignorant" was
occasionally in English from early 14c., but modern use (1823) comes from influence of Ger. dumm. Related: dumber; dumbest. Applied to silent contrivances, hence dumbwaiter
. To dumb down is from 1933.