Fig. experienced; knowledgeable in the ways of the world. I know what's going on. I wasn't born yesterday.Sally knows the score. She wasn't born yesterday.
More experienced and less naive than one appears to be, as in Don't think you can fool me; I wasn't born yesterday. This term gained currency from the title of Garson Kanin's popular Broadway play, Born Yesterday, which was made into an even more popular film. In both, Judy Holliday played a stereotypical dumb blonde who shows more common sense than her sophisticated acquaintances. [Early 1800s]