I was not born yesterday is a saying which literally means that I am old enough to understand everything.
It is another way of telling everyone that I am not stupid or that I cannot be fooled easily.
The person saying this always considers himself mature both in age as well as smartness.
The proverb is generally used in a defensive tone and in response to something said to you indicating your lack of experience. It implies that nobody can take advantage of me so easily.
'I was born on a Friday but not last Friday' is another proverb which means the same.
‘I ‘can be substituted by ‘He' or ‘She' or any other personal pronoun in both the proverbs.
1. When a salesman tried to allure me into buying his product I said, “I will cross check its price first. After all I was not born yesterday."
2. “Why did you lie to your mother about having bunked school today? She was not born yesterday!."
3. Teacher said to the student “you will have to think of a better excuse for not having done the home work. I was not born yesterday!"