born yesterday, not (I wasn't)

Extremely naïve, gullible, or unintelligent, like a newborn baby. Almost always used in the negative or as a rhetorical question. Don't think you can fool me with that old ruse, I wasn't born yesterday, you know. Of course I know that major political issues can't be fixed overnight. Do you think I was born yesterday?

Not naïve or inexperienced; knowledgeable, intelligent, or shrewd. Don't think you can fool me with that old ruse—I wasn't born yesterday, you know. I know you think Mom is clueless about these things, but she wasn't born yesterday.

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]

If you say that someone wasn't born yesterday you mean that they have enough experience to not be easily tricked. Listen, I wasn't born yesterday. This looks like a work of fiction to me. Note: This expression is often varied. For instance, you can say that someone must think you were born yesterday if they treat you as if you are stupid. They must think I was born yesterday if they think I'd fall for a trick like that.

Naive or ignorant. Used in negative constructions: Of course I can use a computer; I wasn't born yesterday.

Not naive; more experienced than one might think. Already a popular saying by the early nineteenth century, it appeared on both sides of the Atlanti “I warn’t born yesterday,” said Thomas Haliburton’s Sam Slick in one of his Wise Saws (1843). Approximately a century later Garson Kanin used the phrase for the title of a Broadway play that became extremely popular, as did the later (1950) film version. In both, actress Judy Holliday played the quintessential dumb blonde who, despite seeming unsophistication, is graced with enormous good sense.