The expression, "I wouldn't piss on him if he was on fire" suggests a very strong hatred for an individual. Often when it is used, someone who hates the person even more comes back with "I wouldn't shit on him if he was on fire", thinking this takes the hate level a step further.

However they are mistaken. Shit, poop or jobbies wouldn't help put the fire out (unlike piss) so in fact by shitting on someone who is on fire you are adding insult to injury, as well as creating a more interesting incident scene for the fire department investigators.

It's important if you are taking a dump on an on-fire person not to let any bits of wee-wee come out as this might put the fire out. Also try to poop from a safe height to avoid charring your back-bottom or your dangly bits/front-bottom. You might also want to try to squeeze it slowly so that the on-fire person can see it coming out and have that as their final vision of life.