The idiom ‘born yesterday’ is often used to describe someone who is naive, gullible, or inexperienced. When you say that someone is ‘not born yesterday,’ you mean they are wise to schemes or don’t easily fall for tricks.
This expression likely stems from the idea that a newborn baby is innocent and lacks the life experience that comes with age. Therefore, if someone is referred to as ‘born yesterday,’ it implies that they are overly trusting or lack awareness about certain realities of life.
For example, if a person tries to sell you a dubious product and you reply, ‘I wasn’t born yesterday,’ you are indicating that you are not going to be fooled easily.