'Dunking' (or 'dunking on') online means to publicly mock or own someone, usually with a witty reply or biting comment. It comes from basketball, where a slam dunk is a forceful, emphatic score. 'He dunked on her' = 'he made her look foolish with a clever comeback'. Generally light-hearted in chat, sometimes nastier in political or culture-war contexts. If your teen 'dunks on' a friend, they are landing a joke at their expense. Worth a conversation if it ever crosses into bullying.