A colorful comparative insult used to describe someone of spectacularly low intelligence. Like its cousin 'dumb as a box of rocks,' it leans on an inanimate object — the doorknob — as the benchmark of stupidity. The humor comes from the deadpan logic: a doorknob performs one simple function and has no inner life whatsoever. Calling someone dumb as a doorknob suggests they may not even clear that bar.
I tried explaining the refund process to him twice, but the guy was dumb as a doorknob and kept asking the same question.
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(simile, slang) Very stupid.
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Regional slang from around the English-speaking world — British, Australian, Irish, Caribbean, Nigerian, Filipino, AAVE, and the hyphenated-English dialects that make the internet sound local.
See all Regional & Other slang on Slangora.