Only Phrases

Meaning
It is used for something which is of highest quality.

Origin
The 1753 edition of Chambers’ Encyclopedia has this phrase under an entry for ‘Diamond’:
“The first water in Diamonds means the greatest purity and perfection of their complexion, which ought to be that of the clearest drop of water. When Diamonds fall short of this perfection, they are said to be of the second or third water, &c. till the stone may be properly called a coloured one.”