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of the first water

Of the finest quality, as in That was a play of the first water. This idiom refers to a grading system for diamonds for their color or luster (compared to the shininess of water). The system is no longer used but the term, used figuratively since the early 1800s, has survived it.

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The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.