It's a word used to describe someone who somehow manages to piss on their feet while standing at a urinal.
*Ben walks into the toilet*
Matt says : "Oh my god, Ben's pissed on his foot again, what a 'piss foot'."
Ben says "Naa.. Rubbish man, it was from the sink, give me a break!"
Matt says "No way, from now on you'll be known as 'piss foot'"