Q: Do you know when the phrase “to test the waters” came to mean “to float an idea.” I can’t help wondering if it once had something to do with “to take the waters,” as at a spa.
A: The expression “to test the waters” (or “water”) has been used literally since the 19th century in the sense of testing water for its purity, chemical content, and so on.
The earliest example we’ve found is from a report in the February 1881 issue of the Canada Medical Record about an outbreak of typhoid fever at Bishop’s College University (now Bishop’s University) in Lennoxville, Quebec:
“It appeared desirable to test the waters qualitatively as to their constitution, as to presence of ammonia or ammoniacal salts, chlorides, and organic matters, also for magnesia.”
The Oxford English Dictionary has a somewhat later citation from The Fatal Three, an 1888 novel by the English writer Mary Elizabeth Braddon: “I have tested the water in all the wells.”
In the 20th century, the expression “to test the waters” took on the figurative sense you’re asking about, which the Merriam-Webster Online Dictionary defines as “to make a preliminary test or survey (as of reaction or interest) before embarking on a course of action.”
The earliest OED citation for this sense is from A Little Murder Music, a 1972 mystery by Diana Ramsay: “ ‘If you’re attempting to establish a motive—’ ‘I’m just testing the water,’ Meredith said.”
And here’s an example from Judith Krantz’s 1980 novel Princess Daisy: “ ‘I guess it’s just a lucky thing that Supracorp’s such a big business,’ Kiki said, testing the waters.”
This example that we’ve found (from Walter Lippmann and the American Century, a 1980 biography by Ronald Steel) uses the expression in the sense of to try out, or stick a toe into, the water to see if its swimmable:
“During the summer of 1898, when with his parents at the resort town of Saratoga Springs, where New Yorkers of all classes retired to test the waters and bet on the horses, he met his first authentic hero.” (The hero was Admiral George Dewey, whose squadron had just defeated the Spanish fleet in the Battle of Manila.)
You asked if “to test the waters” once had something to do with “to take the waters,” a much older usage that the OED defines as “to drink or bathe in the waters from a mineral spring or spa for reasons of health or well-being.”
As far as we can tell, the two expressions have nothing in common except the H2O, which they refer to either literally or figuratively.
The earliest OED citation for the older usage is from The Yorkshire Spaw, a 1652 treatise by John French on four medicinal wells: “I approve not of taking the waters too fast.”
In other words, one should test the waters before taking the waters.
from Grammarphobia http://www.grammarphobia.com/blog/2016/11/test-the-waters.html