MAYOR which is an acrinem for "mosh at you own risk" is common among metalcore/deathcore concerts. It maybe posted somewhere as meaning "if you are hurt while 'moshing' we are not held responsalbe". It is a very new term amongst people and probably will never catch on.
badass 1: Hey do you see that sign it says MAYOR, hahaha
badass 2: Whats mayor?
badass 1:You know "mosh at your own risk"
badass2: Ohh very clever