(That insult was around long before SNL. It goes back to the '40s and implies that someone's mother was a prostitute for the troops.)
(Thanks very much for the origin story, which I didn't know. I never hung out with my WW2 father. I know the kids' insult though, I'm in late middle age now... one the run home to catch DS legions.... childhood was filled with unexplained expressions the troops brought home from the battlefields, like "SNAFU"...)