Hot mom means sexy mom; deduce that to mean mom I'd like to have
sex with;
regard that statement as Mother I'd Like to Fuck, hence the terminology:
MILF.
Joey: Wow, Ross, your ex-wife Carol is one hot mom.
Ross: You're talking about my son's mother.
Joey: Hey, I call 'em as a I see 'em. I'm sure Susan will agree that she's a
MILF.
Ross: Shut up!