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i looked it up, and
from the english wiktionary page “dumb“ ( https://en.wiktionary.org/wiki/dumb#Adjective ), etymology 1, adjective, definition 6:
(informal, derogatory, especially of a person) Stupid.
if you meant stupid, why wouldn’t you say „stupid“? „dumb“ is supposed to mean „un[willing/able] to speak“ (of course, it might be 3at i’m just begging 3e question)