Definition from Wiktionary, the free dictionary
The term originated in the gemstone trade where it was used to signify water-like clarity.
- The highest quality of gemstones, especially of diamonds and pearls.
1880, Cornwall, HB, “Gems and Precious Stones”, in Patterson, WM editor, The Growing world; or, Progress of civilization, and the wonders of nature, science, literature and art, interspersed with a useful and entertaining collection of miscellany, page 20:
- To be the first water a diamond must be absolutely colorless, very lustrous, and perfectly free from flaws.
- (by extension) Of the highest rank or quality.