Mostly used in a joking way, "ur mom" is a gen z word that can be used in many ways. In the ways of saying it box there are many different ways to say it.
SEXUAL WAY-
dude 1- "dude what were u doing last night?"
dude 2- "ur mom"
JOKING WAY
dude 1- "bro what do u want to eat tonight?"
dude 2- "ur mom"