Slang for the beer sold in Utah which has a very low alcohol content. Got the name Mormon Piss Water from an urban legend which states "the mormons drink all the good beer and when they piss afterwards, they bottle it up and sell it to the general public."
Person 1: You drunk yet?
Person 2: No, and this is my 17th beer.
Person 1: You must be drinkin that Mormon Piss Water