Only the most awesome character ever on Code Geass.
Mao was a guy who received a Geass from C.C. that gave him the power to read minds, but because he broke their contract the Geass went out of control and became permanently activated, so Mao heard everyone's thoughts all the time.
Comes from roflmao, an acronym for "roll around on the floorlaughing my ass off"
Therefaore mao is defined as "My ass off".
However it can used for just about everything.