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.