A clever combination of "that's what she said" and "your mom!". It was created to avoid the argument that "that's what she said" creates confusion because "she" is nobody while adding the punch of "your mom" jokes to it.
The phrase "last night" may be added to the end if extra pwnage is needed.
Person 1: "Whoa, that's a big hot dog"
Person 2: "That's whatyour mom said!"
Person 1: "Whoa, that's a big hot dog"
Person 2: "That's what your mom said last night!"
A comeback that people say when someone says, " I bet you are terrible at sex." Or other comebacks. Then that person would come back and say, " That's not what your Mom said in bed last night."
Guy #1: Your dick is probably the size of half of a raisin.
Guy#2: That's not what your Mom said in bed last night.