Our American history teacher in high school told us that none other than Lyndon B Johnson popularized this phrase. Apparently he once referred to some guy as being "angrier than a fly pissed off a toilet rim." I, for one, can imagine that I would be incredibly irate if I was struck by a huge stream of piss so forcefully that I was knocked off my perch, and proceeded to fall into a toilet where the weight of the piss-water on my wings prevented me from being able to even move.