What is Let?
1.
a shortening of the saying "let it be" and also derived from the Norwegian pronunciation of "easy," it means "ease up," "chill" or "take it easy."
"Did that guy just cut me off? I'm going to kill him!"
"Let, sweetie, let. He's just trying to get to the airport on time."
See
2.
Word used in annoyance as a kind of sarcasm.
If you miss a bus: Let me miss that bus.
If hit by a bus: Let me get hit by a bus.
When directed at others:
If someone is annoying: Let you be any more annoying.
No one actaully wants the let to happen, but the common reponse is "Done...".