Let originates from the French word for net which is filet. http://www.wordreference.com/enfr/net
The english upon adopting the game from France changed it like they changed
the scoring and called a let when the ball hit the net on the way in to the
proper service box.
The Americans have changed it even more to mean any hindrence on the court
rather than just the hindrence of the net on the serve. So that is why if a
ball crosses your court while playing a point you call a let, if someone
walks
