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What does the idiom born yesterday mean?

Answer:

It is a way of saying someone is naive - if you were born yesterday, you are a baby with no experience. Usually people say 'NOT born yesterday' to indicate they cannot be fooled.
When people say, "I wasn't born yesterday," they're indicating that they do not believe a story/excuse made by someone else. They are saying that they can see past a given story and see what is really going on.

First answer by Coasterdude. Last edit by Darksyde. Contributor trust: 1164 [recommend contributor recommended]. Question popularity: 3 [recommend question].