–noun
1. any size Medium shirt (especially a T Shirt) worn by a man whose body-size would usually require a size Large shirt for comfort, worn for the accentuating affect it gives to musculature of the torso and arms; The resulting effect is similar to that of a medium-sized person wearing a size Small shirt.
2. a nickname for a man wearing such a shirt
Jeremy - "Rob, come on up here!"
Rob - "What size shirt you got on?"
Jeremy - "Fuck you."
Rob - "The price you pay when you rock a shmedium in this place."
-Season 3 Episode 8 of 'Rob Dyrdek's FantasyFactory'
A sunburn or suntan focused on the left arm. It is caused by the left side of the body being the only half that gets exposed to sunlight while driving. Presumably, it would be located on the right side of the body in countries where they drive on the left side of the road.