The hand you use most often for playing Wii. This hand/arm is more dominant and most of the time, is stronger than the other arm. So you basically wind up with a super muscle-y arm and a weak arm.
The nunchuck arm is always the weakest because it barely has any uses.
Jacob: Hey dude, check out the Wii hand.
Zack: Damn! It's fucking huge!
Jacob: Yeah, been playing Mario&Sonic at the Olympic games. Got buff.
Zack: Your other arm looks wimpy as hell though.