To silence.
"The sudden noise from the alarm clock managed to dumb him before he could even shout out in panic."
slow to learn or understand; lacking intellectual acuity
"so dense he never understands anything I say to him"
"never met anyone quite so dim"
"although dull at classical learning, at mathematics he was uncommonly quick"
"dumb officials make some really dumb decisions"
"he was either normally stupid or being deliberately obtuse"
"worked with the slow students"
Unable to speak; lacking power of speech (kept in "deaf, dumb, and blind").
"After contracting a rare virus, the child remained deaf, dumb, and blind until recovery."
In plain English: Dumb means lacking intelligence or common sense, often used to describe someone who is slow at understanding things.
"The instructions were so dumb that I couldn't figure them out myself."
Usage: Avoid using this archaic sense meaning "unable to speak," as it is now considered offensive when applied to people. Instead, use the word only for its modern common meanings such as foolish or unintelligent.
The word "dumb" comes from Old English, where it originally meant silent or unable to speak. Its modern sense of being stupid likely arose later through contact with related words in German and Dutch.