Meaning of “crazy” in the English Dictionary

"crazy" in British English

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crazyadjective

uk /ˈkreɪ.zi/ us /ˈkreɪ.zi/

crazy adjective (NOT SENSIBLE)

A2 stupid or not reasonable:

It's a crazy idea.
You're crazy to buy a house without seeing it.

mentally ill:

I seriously think she'll go crazy if she doesn't have a holiday soon.

More examples

crazily
adverb uk /ˈkreɪ.zəl.i/ us /ˈkreɪ.zəl.i/
craziness
noun [ U ] uk /ˈkreɪ.zi.nəs/ us /ˈkreɪ.zi.nəs/

crazynoun [ C ]

uk /ˈkreɪ.zi/ us /ˈkreɪ.zi/ informal

a person who is crazy:

There are a lot of crazies out there.

(Definition of “crazy” from the Cambridge Advanced Learner’s Dictionary & Thesaurus © Cambridge University Press)

"crazy" in American English

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crazyadjective

us /ˈkreɪ·zi/

very strange or foolish:

She’s the craziest person I’ve ever met.
[ + to infinitive ] You’re crazy to rent the place without seeing it first.

Crazy can mean mentally ill.

Crazy can also mean behaving in a strange way esp. because of stress, as if you are mentally ill:

The constant whine of the machine nearly drove (= made) me crazy.
I think she’ll go (= become) crazy if she doesn’t take a vacation soon.
crazily
adverb [ not gradable ] us /ˈkreɪ·zə·li/
craziness
noun [ U ] us /ˈkreɪ·zi·nəs/

crazynoun [ C usually pl ]

us /ˈkreɪ·zi/ infml

a person who acts in a strange or threatening way, esp. one who is mentally ill:

Are we talking about a bunch of crazies or about a legitimate military force?

(Definition of “crazy” from the Cambridge Academic Content Dictionary © Cambridge University Press)