dumb: [OE] The notion underlying dumb is of ‘sensory or mental impairment’. It goes back to a nasalized version of prehistoric Indo-European *dheubh-, denoting ‘confusion, stupefaction, or dizziness’, which was also the ultimate source of English deaf. This developed two stands of meaning. The first, through association of ‘sensory or mental impairment’ and ‘slowwittedness’, led to forms such as German dumm and Dutch dom, which mean ‘stupid’ (the use of dumb to mean ‘stupid’ did not develop until the 19th century, in American English, presumably under the influence of the German and Dutch adjectives).
The other was semantic specialization to a particular sort of mental impairment, the inability to speak, which produced Gothic dumbs, Old Norse dumbr, and English dumb. (The German word for ‘dumb’, stumm, is related to English stammer and stumble, as are Dutch stom and Swedish stum.) Dummy [16] is a derivative; it originally meant ‘dumb person’. => deaf, dummy
dumb (adj.)
Old English dumb "silent, unable to speak," from PIE *dheubh- "confusion, stupefaction, dizziness," from root *dheu- (1) "dust, mist, vapor, smoke," and related notions of "defective perception or wits."
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. Meaning "foolish, ignorant" was occasionally in Middle English, but modern use (1823) comes from influence of German dumm. Related: dumber; dumbest.
Applied to silent contrivances, hence dumbwaiter. As a verb, in late Old English, "to become mute;" c. 1600, "to make mute." To dumb (something) down is from 1933.
