of the first water

中学生でも知っている単語での表現なのに意味がすっきりとは分からないことがあります。My Fair LadyでHIGGINSが次ぎの台詞で "the first water" と言っていますが、何でしょう？
HIGGINS: By George, there's a man for you! A philosophical genious of the first water.
辞書を見ます。
・Collins Dictionary: the highest grade or best quality: Besides, during the few weeks she had been at the school Jocelyn had proved herself to be a leader of the first water.
・American Heritage Dictionary: The foremost rank or quality: a pianist of the first water.
上記の意味になったのは "the first water" の "the highest degree of fineness or purity in a diamond or pear." の意味から来たものらしい。