Entering a McDonalds, or other fast food joint for sole purpose of using toilets.
Similar to McShit
Am bustin' here, need to go for a McPiss.
What u gotta do after drinking an extra large soda from mcdonalds.
Brad: Man I gotta McPiss! I shouldn't have gotten the extra large!!
Jordy: ya dumbass. Why'd you get it anyways?
Brad: it was a good deal! I couldn't just not get it!!!
Jordy: fucktard. *walks away*