[Q] From Kery Gray: “We had just concluded practicing for a major presentation when my boss’s boss popped off this question: ‘Why is a dry run dry?’ We all agree that a dry run is a practice session but haven’t a clue how the phrase came about.” (来源：英语学习门户网站EnglishCN.com)
[A] The sense of rehearsal is known in the United States from the early 1940s. The oldest example I can find is from the Gettysburg Times for August 1941 in reference to an army operation: “The occasion was a ‘dry run’ for the maneuvers that will begin within the next ten days.”
One explanation that is often given is that it is linked to a much older North American sense of an arroyo, a stream bed that is normally dry or almost dry but which floods after heavy rain. These are common in the USA, as witness the many places called Dry Run. (Run here just means a course or route.) This sense dates back to the 1840s. One might guess that the idea behind the rehearsal sense is that it’s like a dry river bed before a storm, in waiting for the big event when the rain comes and it fulfils its potential function.
However, that explanation seems too great a stretch of meaning. (Incidentally, I’ve also come across a story, in Brewer’s Dictionary of Phrase and Fable, that it refers to “the reconnoitring by bootleggers of the route they plan to use before transporting their illicit goods along it”. That’s so stretched it broke the elastic, not least because the expression dry run is definitely older than Prohibition.)
There I had to leave matters until Douglas Wilson of the American Dialect Society found evidence for a much more plausible origin.
The term run, more fully fire run, has for at least the past century been used by local fire departments in the USA for a call-out to the site of a fire. It was once common for fire departments or volunteer hose companies to give exhibitions of their prowess at carnivals or similar events. A report of one such appeared in the Stevens Point Journal for 8 July 1899: “Wednesday night’s carnival feature was a grand exhibition fire run by the Milwaukee fire department, under the direction of Fire Chief James Foley.” Companies also competed with each other to show how well they could do. These competitions had fairly standard rules, of which several examples appear in the press of this period, such as in the Olean Democrat of 2 August 1888: “Not less than fifteen or more than seventeen men to each company. Dry run, standing start, each team to be allowed one trial; cart to carry 350 feet of hose in 50 foot lengths ...”.
These reports show that a dry run in the jargon of the fire service at this period was one that didn’t involve the use of water, as opposed to a wet run that did. In some competitions there was a specific class for the latter, one of which was reported in the Salem Daily News for 6 July 1896: “The wet run was made by the Fulton hook and ladder company and the Deluge hose company. The run was made east in Main street to Fawcett’s store where the ladders were raised to the top of the building. The hose company attached [its] hose to a fire plug and ascending the ladder gave a fine exhibition.”
It’s clear that the idea of a dry run being a rehearsal would very readily follow from the jargon usage, though it first appears in print only much later. Douglas Wilson found that by March 1943 the idea of a dry run as a rehearsal had so taken hold that Stars and Stripes created an odd-looking compound term in a feature on an airbase crash team: “There aren’t any brass poles, and no false alarms, but there is plenty of authentic firehouse atmosphere around the place. Regularly the crash crews go tearing out on a dry run; once in a while they empty the 400-gallon tank on their truck in a wet dry run.”
He also points out that there is another sense in which dry run is used today in the US — that of a call-out of an emergency service, such as an ambulance, in which no service is given, either because the patient refused help or because no emergency was found. He suggests that this might have arisen through an extension of the firefighting term in situations in which the crew arrived at the scene of a supposed fire but found either that it was already out or that it was a false alarm. In neither case would any water be pumped, so they were also dry runs in the firefighters’ jargon.