A contraction of steroid head, if you describe someone as a "sted" it means they are very strong, not on steroids (necessarily). Often used sarcastically. Generally used in the UK.
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.