Usage patterns
間抜けな〈人〉a foolish/idiotic person
He's thought of as a foolish guy.
間抜けなミスをするto make a stupid mistake
I made another stupid mistake.
間抜けな顔をするto have a blank, foolish look on one's face
He had a dumbfounded look on his face after being surprised.
〈人〉は間抜けだsomeone is a fool/idiot
He's an idiot for coming up with a plan like that.