What does flap stand for in English?

"Flap" is not an acronym, so it doesn't stand for anything. It's a word in itself with various meanings, including:

* A piece of cloth or other material that hangs loosely: Like the flap on a tent or a bird's wings.

* A sudden, quick movement: Like a bird's wings flapping or a heart flapping in your chest.

* To move quickly and irregularly: Like a flag flapping in the wind or a child flapping their arms excitedly.

It can also be used in other contexts, like "to flap your gums" (to talk excessively).