English definition of “why”
› used to ask or talk about the reasons for something: Why didn't you call me? I wonder why he didn't come. So that's the reason why he asked her!
Why don't you?/Why not do sth? › used to make a suggestion: Why don't you come with us? Why not give it a try?
why not? informal › used to agree with something that someone has suggested: "Let's have an ice cream." "Yes, why not?"