Dumb
Old English, of Germanic origin; related to Old Norse dumbr and Gothic dumbs ‘mute’, also to Dutch dom ‘stupid’ and German dumm ‘stupid’.
wiktionary
From Middle English dumb(“silent, speechless, mute, ineffectual”), from Old English dumb(“silent, speechless, mute, unable to speak”), from Proto-West Germanic *dumb, from Proto-Germanic *dumbaz(“dull, dumb”), from Proto-Indo-European *dʰewbʰ-(“to whisk, smoke, darken, obscure”).
The senses of stupid, unintellectual, and pointless, which are found regularly since the 19th century only, probably developed under the influence of German dumm and Dutch dom. Just like the English word, these originally meant "lacking the power of speech", but they developed the mentioned senses early on.
From Middle English dumben, from Old English dumbian (more commonly in compound ādumbian(“to become mute or dumb; keep silence; hold one’s peace”)), from Proto-Germanic *dumbijaną, *dumbōną(“to be silent, become dumb”), from Proto-Indo-European *dʰewbʰ-(“to whisk, smoke, darken, obscure”). Cognate with German verdummen(“to become dumb”).
etymonline
dumb (adj.)
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.
dumb (v.)
late Old English, adumbian, "to become mute, be silent, keep still," from dumb (adj.). From c. 1600 as "to make mute." Related: Dumbed; dumbing. To dumb (something) down "make less intellectually challenging, make simpler to understand" is from 1933.
