Obviously, to fall flat is exactly that. However, as an idiom it refers to either a joke or some piece of advice or 'wise words' that nobody properly understands. So you say these words and everybody looks at one another and says "So what?" It will have fallen flat.
This is not an idiom. When you see AS ___ AS ___ you are looking at A Simile. This is comparing two flat things.
If your plans fall through, it means they did not work out.
It means to fall asleep while sitting (or standing).
It means to fall face flat.
fall a prey
It means to either fall actually on your face or to just fall. It could also mean someone screwed up.
The idiom "to fall a prey" means to become a victim or to be taken advantage of by someone or something. It implies being trapped or helpless in the face of a situation or a person's actions.
yes, it is.
This is the equivalent of our American idiom: If you fail, try try again.
It's not an idiom - it's a description of what happens when you get nervous and excited. Your legs feel wobbly and you might even fall over because your knees get weak and collapse underneath you.
It is an idiom meaning that someone who had stopped drinking alcohol has slipped up and started drinking again.
Flat busted, broke, penniless, can't rub two nickels together, tapped out, all are examples of ways to say you have no money. That you're poor or without funds.