The English language words Stupid, Silly and Dumb have related meanings, but they are all different, as I shall explain, based on common British English usage.
Stupid means of low intelligence.
"The bank robber was stupid to remove his mask in the building."
"This test is stupid, there is no right answer."
Dumb is an archaic &/or very informal way to say the same as stupid. Dumb was also used in the past to describe someone who could not speak, so using this word as a way to describe someone with low intelligence is offensive and should not be used.
"You are so dumb, just stop spending so much money."
Silly means to behave in a way that is not normal or correct, often with an element of comedy.
"My mum is so silly, she decided to make a milk shake out of paint."
"I love my dog's silly glasses."
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