not all there

(idiom) a little bit crazy, not thinking properly

Example Sentence:
Mom: If you think I’m going skydiving, you’re not all there.
Son: Come on, mom!
Mom: No way!

If Steven thinks he can go out on a date with the most beautiful woman in school, he’s not all there.

From the Blogs:

Here but not all there

Media:

Similar idioms:

1. got a screw loose
2. off one’s rocker
3. see someone