It's as if you were washing something away and it drained down the sink or bathtub drain. It's gone, it's not coming back, and there's no sense trying to get it back.
Do what ever you want to do.
The water goes down the plughole/drain and then down the pipes.
it means go to sleep
It means to go faster
The sewer is where all detergent goes when it goes down the drain. Any type of liquids which go down a drain will go into the municipal sewer lines and may end up in treatment plant.
All of them?
it means to take a chance or risk
It is the drain.
This is not an idiom. An idiom is an expression whose meaning cannot be deduced from its elements. To go through fire and water for someone is easily understood to mean to make a special effort and to undergo difficulties for that person's sake.
It goes down the toilet but if you go in the sink it goes down the drain.
I'd say that's more of a true statement rather than an idiom. Prices never DO come down - they always go up.
Down the drain