'Don't you know what you mean to him'. It's a phrase from HYYH The Notes. Yoonkookers (and sometimes non-Yoonkookers) use it to hurt each other's feelings on Twitter.
Yoonkooker 1: DYKWYMTH?
Yoonkooker 2: *quietly breaks down into tears*
Fuck boy ass ugly ass boy, the type to talk shit about his ex but continues to text and calls her. Always trynna fuck.. stay trying to shoot his shot with any girl knowing that he ugly as hell. Nobody likes a dykwan..