It's a United pilot tradition that goes back at least to the 90's: the 8 ball is a term of endearment for the most junior person in any given class. There's a ceremony one night during basic indoc week where the week's junior person is given a bowling ball-sized 8 ball and has to carry it with them everywhere they go. It's all in good fun with drinks and good cheer. When the next class comes along, the ball is passed on.
God save the ball!