Tap dancing is what people do when they are absolutely bursting for a piss, to the point of not knowing whether they will be able to prevent wetting their pants. People who are tap dancing jump about on the spot, and then start
Jigging up and down and waving hands and moving back and forth
all help to prevent an embarrasing gush of piss flooding out.
Ohhh maaan I cant get on the bus if it comes now, I been tapdancing for ten minutes and it wont go away.
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.