english
1
general::
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.
American Heritage Idioms
2
general::
Phrase(s): of the first water Fig. of the finest quality. • This is a very fine pearl—a pearl of the first water. • Tom is of the first water—a true gentleman.
McGrawhill's American Idioms And Phrasal Verbs