Idiot
noun, な adjective
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.
まぬけ
The readings come from the ま reading of 間 and from 抜く.
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.