French expletive, litteraly "Go fuck your mum".
Mostly used by kids as an act of rebellion against someone they hate, or simply as an expression of their disagreement/anger. Most of the time though, they don't realize what they're saying since they're merely imitating the wechs they heard, or they're running out of insults.
Kid #1: va te faire foutre!
Kid #2: j'temmerde!
Kid #1: PD sans couilles! (literally "ball-less homosexual", lmao.)
Kid #2: nique ta mere fils de pute!