An acronym of Kiss My Teeth. Basically, when you dissaprove of someone, or you want to show your dislike of them, you sort of put your tongue over your front teeth and make a little kissing noise (whilst giving them a dirty look). So writing KMT is telling the person that you don't like them.
Girl 1- Your such a slut! KMT.
Girl 2 - Shutup you bitch!