normally expressed as an intended slight on said maternal figure. often serves as indication of the end of a conversation.
Y: You're a fat farm animal.
X: I'll show you a fat farm animal
Y: You're going to be a fat farm animal if you dont shut up
X: Oh yeah, your mom's a fat farm animal OHHHHH!
Not only is the useage commonly meaningless and inaccurate, it's a piss-poor and half-assed attempt at trying to offend someone who is clearly clever and better at insulting you than you are at insulting them.
The term is also pretty annoying, petty and pitiful.
Person #1: I see that you are overweight, unlikeable and have a horrid personality... Who taught you to be the way that you are, you skanky slut?
Person #2: Your MOM!
Person #1: I suppose that makes sense, since you've never met with my mother or have ever been acquainted with her. Whatever. You suck at making insults.
Person #2: Um... shit, I guess you're right.
2. An all-purpose insult, used to end a conversation, cover for the fact that you can't think of a witty comeback, or pwn somebody. When used in this sense, the implication is that your mom is sexually promiscuous, of below-average intelligence, grossly overweight, or any combination thereof. Does not necessarily have to make a lot of sense, e.g. 'Your mom goes to college!'
3. A ho.
Jane: 'Your mom!'
Jane: 'Ew, this pizza tastes weird.'
Dick: 'Your mom tastes weird.'
Dick: 'Where's the remote control?'
Jane: 'I left it in your mom.'
Stephen Hawking: 'The whole history of science has been the gradual realization that events do not happen in an arbitrary manner, but that they reflect a certain underlying order.'
Dick and Jane: 'Your mom doesn't happen in an arbitrary manner!'
Dude: "Yeah so what's the homework tonight?"
Dude 1: "Your mom, bitch"