"Ur mom" Is a known slur amongst youngsters now-a-days.
Tim: Hi
Tommyinnit: Ur mom!!
It's almost like saying "fuckoff you dumbass"
ur mom is a very offencive word to win any fight
personA: yeah well fuck you
personB: yeah well fuck ur mom
(walks off cooly) (other persons jaw drops) personB: yeah dont mess with me
This is what a dumb nerd says whenever someone insults them
Normal human: ur so bad
Dumb nerd: Yea well, Ur MOm
The person I fucked last night
Person: where we're you last night?
Me: In ur mom's bed.