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/
person 1: 'i got a f in maths'
person 2: 'see your life mtchew'