A plastic mattress protector; used to prevent a mattress from becoming wrecked if somebody has enuresis (if they piss the bed). They are often found on beds at group homes and similar facilities.
Robbins then thinks about the pissblankets on every bed in the joint, so he rips the pissblanket off of one of the beds, goes around and unscrews all of the light bulbs he can get to, pours the generous helping of light bulbs into the pissblanket (approx. 24 bulbs), gathers up the edges, and whirls the bulging pissblanket over his head -- striking the wall with it a number of times until he is absolutely, positively, 100% certain that all of the bulbs inside are busted. He then crams the pissblanket and its cargo of broken bulbs into a nearby wastepaperbasket.