dumb – немой, безмолвный, онемевший, беззвучный, скрывающийся

dumb (adj.) (староанглийское «dumb» - немой); из PIE *dheubh- смущение, замешательство, оцепление, остолбенение, головокружение; из корня *dheu- (1) – пыль, пар, дым; нарушение восприятия или разума.
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.

Это опять «тупой», «дуб», «дубина». См. «deaf». В нашем случае «um» - «юс» (назальный звук).