In conversation and informal writing, people often describe a foolish action or idea as mad.
Camping in winter was a mad idea.
You would be mad to refuse such a great offer.
In conversation, mad is sometimes used to mean ‘angry’. If you are mad at someone, you are angry with them.
When she told him she wouldn't go, he got mad.
My parents were mad at me for waking them up so early.
If you are mad about something that has happened, you are angry about it.
He's really mad about being lied to.
In conversation, you can say that someone is mad about an activity, when they like it very much.
Her daughter is mad about dancing.
The whole family is mad about football.
If someone has a mental illness that makes them behave in strange ways, don't say that they are ‘mad’. You should use the phrase mentally ill.
She spent time in hospital when she was mentally ill.
The drug is used to treat mentally ill patients.