When one arm is grotesquely more muscular than the other.
Rafael Nadal has such a Wanker's arm
1) When the muscle of one arm is pumped up due to too much masturbation.
2) When one arm is aching because of too much masturbation.
3) When one arm is aching due to the repeated performance of a task such as sanding down a piece of wood.
Joe: I am a chronic masturbator. I suffer from Wanker's Arm.
Use 1 - Where you wank too much and your muscle on that particular arm is build up a lot.
Use 2 - An affliction, because of over-use it hurts when you are using it.
Example 1 - Whoa.. look at my wanker's arm - HUUUGE!
Example 2 - Ow.. hmm wanker's arm! *carries on anyway*