In America, we'd say something is "crazy" if it's unbelievable or surprising (or indeed crazy). In London, the equivalent word is "mad". As in, "He's gone mad!" or "That's absolutely mad, man!"
Just heard this song, yo! It's a MAD TING! An absolute mad ting.
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.