Backwash

What does mean by "backwash"?


Thanks in advance.

Every time you take a drink of something, a small bit of saliva leaves your mouth as the liquid returns to the bottom of the bottle, glass, cup, or whatever. That is called "backwash."

If your glass is almost empty and you offer someone a sip of it they may say "no way, all you've got left is backwash."

I always thought this referred to cleaning out swimming pool filters and pumps, etc. by flushing water through them backwards.

It can mean both of these things. Skippy's answer is a little more colloquial. It can also mean the water from a breaking wave as it flows back away from the beach, and more generally just when a fluid goes backwards, or in some sense the other way to that intended.