The hand you piss with. Usually the right hand, though some have been known to "pull a lefty" for amusement. Note that using the non-preferred hand can lead to pissing on your shoes and/or other vital organs.
The advantage of having one chosen piss hand is that only one hand need be washed after pissing.
Ricardinho: Hey, dude, which is your piss hand?
Albertinho: Same as my jerk hand: the right.