born yesterday

[born yesterday] {adj. phr.} Inexperienced and easily fooled; not alert to trickery; easily deceived or cheated. - Usually used in negative sentences.

When Bill started the new job, the other workers teased him a little, but he soon proved to everyone that he wasn't born yesterday.

I won't give you the money till I see the bicycle you want to sell me. Do you think I was born yesterday?

Compare: NOBODY'S FOOL.