A UBG (Unwanted Birthday Gun) is what an American man in the West or South receives on his birthday if he is not interested in hunting or target shooting.
"A Ubg mf" (An Untitled Boxing Game Motherfucker) meaning;
A person who crossdresses, is acting like the opposite gender, might be homosexual or is just a salty/unmeaningful person
"You don't play fair, man! You always just do the same move on me!" - Person 1 "Oh, you a ubg mf." - Person 2
A person who sends writes letters, post cards, greeting cards, text messages etc. to him-/her-self, and pretends that the correspondence is from someone else.