what is "has a screw loose" mean?

[stephannie]

English Idioms and Expressions, Intermediate level

ESL/EFL Test #20 "English Idioms and Phrases", question 10

I would stay away from that guy if I were you, he has a screw loose.

(a) is dangerous
(b) is annoying
(c) is a flirt
(d) is crazy

English Idioms and Expressions, Intermediate level

ESL/EFL Test #20 "English Idioms and Phrases", answer 10

I would stay away from that guy if I were you, he is crazy.

Correct answer: (d) is crazy
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what is has a screw loose mean?

stephannie

[Lost_Soul]

Hi :)

If you have a screw loose then you're out of your mind