Idiom
know better than toknow better (than to do something)
1.
to be wise enough to behave in a more responsible or acceptable way.
It was a stupid thing to do, and I thought she knew better.You know better than to interrupt when someone else is talking.
Usage notes: sometimes used in the form not know any better:
Doesn't she know any better than to wear jeans to a place like this?
2.
to be wise enough to know that you should not believe someone or something.
The guy who was trying to sell us the car said it was in great condition, but I knew better.