WordReference Random House Unabridged Dictionary of American English © 2020
first water, 
  1. Jewelry(formerly) the highest degree of fineness in a diamond or other precious stone. Cf. water (def. 13).
  2. the finest quality;
    highest rank.
  • 1745–55

Collins Concise English Dictionary © HarperCollins Publishers::
first water n
  1. the finest quality of diamond or other precious stone
  2. the highest grade or best quality
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