It’s an expression used to talk about someone who thinks he knows everything. In other words, a person who tries to impress people with his knowledge and ends up annoying them.
So when you refer to someone as a `know-it-all’, you are not really complimenting the person. You are, in fact, showing your disapproval.
Examples: I don’t wish to speak to that know-it-all. * “Saraswathi, the know-it-all in our family, told us to invest our funds in the stock market.
It’s also possible to say, `know-all’. They both have the same meaning.
Examples: Nobody really likes Chandra because she is such a know-all. * “Our class is full of know-alls.