expression, noun-modifying (irregular)
Prenominal phrase describing a person or action as foolish, slow-witted, or lacking common sense. Often used in a mildly insulting or exasperated tone.
Don't ask such a stupid question.
He stood there with a vacant look on his face.
Derived from the noun 間抜け (まぬけ, 'fool') by inserting the possessive particle の and using the past/perfective form of 抜ける (ぬける, 'to be missing'). Literally 'having a gap missing', implying a lack of wit or intelligence.