Idiot
Dumb
noun, な adjective
You learned that 抜ける is "to fall out." If an interval of time falls out of you, you're totally spaced out. You won't learn anything and you'll become an idiot.
間抜け can refer to both a person who does something foolish and to the foolish actions themselves. It's often used to criticize dumb behavior.
The readings come from the ま reading of 間 and from 抜く.
オレ、間抜けだから、気づかなかったよ。言ってくれてありがとう。
I'm too dumb to have noticed. Thanks for telling me.
この間抜け野郎！
You idiot!
間抜けな返事をしてしまい、はずかしく思っています。
I feel ashamed of the stupid reply I sent.
アイツの間抜けヅラ、見た？
Did you see his dumb face?
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.