Noun: A hard disk shaped crust of fecal matter that forms in the anus after each defecation. This is an extremely important part of a bowel movement in that it keeps ones anus from constantly leaking liquid shit. This disk shaped piece of shit acts as an anal dam but it can be broken at great embarrassment to the host of the anus. Wrestling, Yoga and/or laughing too hard can all cause enough stress to rupture a pustule.
Did you see that trail of shit? Beirne must have broke his pustule.