A mechanic's arm is what happens when you wank too much; one arm becomes more muscular than the other. Someone who lies on their back all day tightening nuts.
John: "Jaidon, you've probably got mechanic's arm from her Facebook photos".
Jaidon: "Ha ha, very funny."
Jaidon: "Ha ha, very funny."
by Phrewb October 31, 2011