"dumber" in the adjective sense
1. dense, dim, dull, dumb, obtuse, slow
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"- Thackeray
"dumb officials make some really dumb decisions"
"he was either normally stupid or being deliberately obtuse"
"worked with the slow students"
2. speechless, dumb
temporarily incapable of speaking
"speechless with shock"
lacking the power of human speech
4. dumb, mute, silent
unable to speak because of hereditary deafness
Source: WordNet® (An amazing lexical database of English)
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