When backed into a corner, this phrase functions much in the same way as the phrase, "fuck you and the horse you rode in on". Taking a more aggressive stance, this phrase entails the classic notion of giving your adversary a dirty punch in the brown eye, but one for his/her mother as well, offering a powerful alternative to the default "horse you rode in on". Though bringing bestiality into an insult is tandem to a wild-deuce, changing the horse to the culprit's mother is a straight-flush deal that can be used in any critical circumstance to let your friend know exactly how you feel about him.
Friend: Oh... oh Jesus! Those are my numbers... I JUST WON 98 MILLION DOLLARS!!
You: No shit? Right on man! Hey, do you think I could get a pinch off of those bucks? I mean, I WAS the one who suggested which numbers to pick.
Friend: You're absolutely right. $100 of the jackpot is yours, bro!
You: K. Fuck you AND your mother, jack off.