noun
Derogatory, colloquial term for a stupid person. Often used in a mocking or dismissive tone, similar to 'dumbass' or 'blockhead'. Not extremely vulgar but clearly insulting.
あんなすっとこどっこいの言うことなんか聞くな。
Don't listen to what that idiot says.
お前は本当にすっとこどっこいだな。
You really are a moron.
The exact origin is uncertain. It may be a playful, emphatic extension of すっとこ (a dialectal or archaic term for a fool) combined with どっこい (an interjection or meaningless emphatic suffix). The word has a comical, exaggerated sound typical of Edo-period slang or theatrical language.