Definition of "first water"

first water

noun
uncountable
1. The highest quality of gemstones, especially of diamonds and pearls.
  Quotations
  To be the first water a diamond must be absolutely colorless, very lustrous, and perfectly free from flaws.
  1880, HB Cornwall, “Gems and Precious Stones”, in WM Patterson, 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
2. (by extension) The highest rank or quality or the greatest degree.
  Quotations
  “A nice, juicy steak,” he is said to have called for, “French fries, apple pie and a cup of coffee.” It is probable that he really said “a coff of cuppee,” however, as he was a wag of the first water and loved a joke as well as the next king.
  1922, Robert C. Benchley, chapter XXII, in Love Conquers All, Henry Holt & Company, page 111
  Presumably this was another case in which an engine had failed and had been replaced at short notice; certain it is that none but experts of the very first water could have coaxed such amazing work out of an engine of such comparatively small dimensions.
  1934 February, Cecil J. Allen, “British Locomotive Practice & Performance”, in Railway Magazine, pages 93–94