Actually, in vietnamese, it is written as "Dau ma may" which translates literally to: "your mom's head". So I guess it is an insult to your mom, and to you indirectly.
DUDE 1: I went to the library after school. That's why I'm home late.
DUDE 2: Dau ma may(DU MA MAY) you went to the library after school. Fucking liar.