"Water" has been like that for me. Certain expressions have been coming up and I started to get them confused in my head (if I was ever clear on them). For my own clarification, I just wanted to render these here so I don't get them mixed up again.
- Remote or insignificant.
- Reservoir of water welling up behind an obstruction.
- (of the) first water
- Of highest value or purest quality.
The second definition seems clear once you see it spelled out like that, yet I don't know how I would have ever come across this term in casual conversation. No one I know has ever been overly concerned with "the section which is influenced by the conditions at [a river's] mouth."
The third has a pedigree as rich as usage by Shakespeare, and it apparently refers to the clarity of a diamond. The purest diamond should be as clear as a drop of water, then an ordinal value is added to rank the diamond from "highest quality" to "colored stone."
Not included is "greywater," about which I am absolutely not confused. This is a catch-all term for dirty, used water produced by a domestic environment, everything from laundry water to what you flush down the ter-let.