when someone says
something so unreasonably foolish and stupid theres no way to reply to the statement they have just said so you smack your lips making a 'mtchew' noise. it is also used to express annoyance,
disappointment or some cases used to show in a friendly way distaste for a sarcastic remark. it is sort of a 'ugh' for
black people or used as a eye roll but as a word. the word is when your tongue is sucking your teeth/