'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*
dyktmm
2:The Muffin Man?
2:Yes, I know the Muffin Man .
2:W-Who lives on Drury lane?
1:Well , she’s married to The Muffin Man.
2:The Muffin Man?!
1:THE MUFFIN MAN!!!!!
2:She’s married to Th muffin Man...