A dry run is a rehearsal. The term appears to come from American firefighting jargon, where dry denoted a practice where the hoses were not turned on.
Run, meaning a response to a fire alarm, either real or a drill, dates to the late 19th century. The Portland Morning Oregonian of 11 September 1886 contains a use of wet run in reference to a contest between fire companies:
Open to all; wet run; distance, 200 yards to hydrant; lay 350 feet of hose; [...] ; attach pipe and throw water.
Dry run appears in the New York Times of 14 August 1893:
For the dry run, standing start, one trial will be allowed.
And there is this detailed description of a dry run from the Frederick, Maryland News of 28 May 1901:
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 250 feet of hose, in 50-foot lengths; distance, 200 yards to hydrant, attach and lay one line of hose 150 feet from hydrant, break coupling and put on pipe,...ready for water.
The journal American Speech records dry run as Army slang in 1941. Army use in WWII probably accounts for the widespread use of the term in the general population.
And there is this use of wet dry run in military firefighting parlance in 1943 that indicates dry run had lost its association with water by this date. From Stars and Stripes, 17 March 1943:
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.
(Wordorigins.org regular Douglas Wilson did the research here, turning up the early firefighting uses.)
Copyright 1997-2013, by David Wilton