It is an overly used gay joke that disses someones mom. Came from the show Yo Mamma
Yo mamma is so stupid she was talking into a letter trying to send voice mail.
Yo mamma is so fat the last time she saw her phone number was on a weight scale.
Yo mamma is so fat that when she was sitting in the road i tried to swerve
around her but ran out of gas
Yo mamma is so fat the when she walked past the TV i missed 3 episodes of my show.