Noun and な adjective
まぬけ
彼に間抜けな田舎者のお上りさんだと思われたくないの。
I don’t want him to think of me as a stupid country bumpkin.
抜く is to extract. If you extract yourself for an interval of time from the world, you won't learn anything and become an idiot.
The readings come from the ま reading of 間 and from 抜く.