A lame joke, pun, or play on words that your mom would say, such as "have a nice trip, see you next fall". Does not have to be told by your mom, just has to be lame.
"Hey Trevor, have a nice trip, see you next fall! Haha!"
"Dude, that was fucking lame."
"Yeah man, that was a total mom joke."
A mom joke is when someone makes a joke and laughs really hard at it but no one else does. It's pretty much PAST the "fake laugh joke" point.
Bob: Hey hey what did the kids say to the rabbi? Silly rabb-
Joe: Say another fucking mom joke and I'm gonna slit your throat.
When a hilarious joke is told only to be followed by a lame attempt to make it funnier by useing a cheesy comparison. Usally made by a mom/mother.
P1: Why did the chicken cross the road?
P1: to get the the other side
Mom: Haha like a turkey
Everyone else: O_O ugh mom joke