Six Words For Stupid: 蠢笨傻戆愚钝

In Cantonese there are six words for stupid. Each has a slightly different shade of meaning.

蠢 [cheun]
Stupid. Can also refer to worms wriggling.
Example: 蠢才 [cheunchoy], literally "stupid talent / talent for stupidity / stupid one".

笨 [ban]
Stupid. Can also mean foolish, dull, or awkward.
Example: 笨蛋 [bandaan (Cantonese); bendan (Mandarin)], literally "stupid egg". More often used in Mandarin than Cantonese.

傻 [so]
Silly. Also carries the meaning of stupid, foolish, or imbecile.
Example: 傻婆 [sopo], literally "silly old woman".

戆 [ngong] stupid, simple-minded
Stupid. Can also mean simple-minded in the negative sense.
Example: 戆居, 戆戆居居 [ngonggeui, ngongngong geuigeui], which means "stupid". Also used in Hokkien where 戆 is pronounced [gong].

愚 [yu]
Foolish, stupid, or doltish.
Example: 愚公移山 [yugong yisaan], a four-character idiom directly translated as "foolish old man moves mountain", but the actual meaning is "persistence overcomes mountainous obstacles". As with all these Chinese idioms you have to know the story behind to get the actual meaning, because these idioms make no sense when directly translated.

钝 [deun]
By itself, it means blunt, or dull as in a worn blade of a knife. It can also mean dimwit, dim-witted or retarded.
Example: 迟钝 [chideun], which means "retarded". Can be used for mentally retarded or developmentally retarded.