Girl: Rip, mother, I love you.
Me: Sorry for your loss.
Girl: Stop, I have a boyfriend.
Me: Stop, I have a mother.
Girl: Rip, mother, I love you.
Me: Sorry for your loss.
Girl: Stop, I have a boyfriend.
Me: Stop, I have a mother.
Listen here, dummy. The joke plays on the double meaning of "stop." The girl uses "stop" to mean "cease" because she has a boyfriend. The comedian uses "stop" as an imperative command to mean "halt" because he also has a mother.