PJ Share definition Flag (piss jug) a bedside empty bottle (gallon-sized, liter-sized, etc.) in which you can take a leak in when you're too lazy to get out of bed. Dude it's time to get rid of that gallon of piss next to your bed. No way dude, its my PJ. PJ by scottbalzer December 14, 2010 0 0 Get merch Get the PJ mug. Buy Now — $32.95 See more merch