Standing for "Unidentified Party Bruise".
the common (but not remembered) cause for this is :
a) running into things
b) falling on your face
c) general clumsiness
These contusions are noticed most likely the morning after, with a slightly hungover expression of "huh?"
"Sh**, I totally don't remember getting this U.P.B"
"I thought you punched me in the face, but now I find out I just fell over"
"There is a huge U.P.B. on my leg"