a B-allsack is a variation of the word ballsack. virtually having no meaning but whatever you want it to be. Other uses are "Blue allsack, bigass bus sack, B L smallsack. B L tallsack, Biggie Smallsack, Bearenstein Ballsacks, Humpty Dumpty Sat on the wallsack, etc. Asking someone if they would like a Belgium Wallsack.
B-allsack is a name you say to someone who use basically mwaning, wtf dude, your gay, etc. the tone really emphasizes the meaning.
did you see that B L tallsack riding in the blue allsack? what B-Allsack.