an abbreviation of the term "bustin my balls."
similar to: grinding my gears, busting my chops, etc.
also much more widely usable in public, and around formal guests. Most appropriately used when someone is "telling you their life story" or any other completely unnecessary information that is annoying and/or unneeded.
Marsh: "I love this band."
Luke: "Did you know that the lead singer was once addicted to basket weaving? When he was 13...
Marsh: "Dude. Luke. you're B'in my Bs bro"