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 extrapwnage is needed.
Person 1: "Whoa, that's a big hot dog"
Person 2: "That's what your mom said!"
Person 1: "Whoa, that's a big hot dog"
Person 2: "That's what your mom said last night!"
The best joke ever. Soon to replace the terrible "that's what SHE said" jokes. Anything mildly anti-Semitic, involving cooking, showers, or gas breeds necessity for this comment.
"You stink, man. Go take a shower!"
'That's what Hitler said!'
A variant of "that's what she said." Instead of referring to sexual connotations, it is used to refer to spending a lot of money, making a mess, or fucking up very badly. Arose after the 2010 BP oil leak in the Gulf of Mexico.
X: Oh man, I really screwed the pooch on that one.
Y: That's what BP said!
X: It's going to take me all day to clean this mess up.
Y: That's what BP said!
X: I'm paying tonight. The sky is the limit.
Y: That's what BP said!
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.