early 13c., from O.Fr. duble, from L. duplus "twofold," from duo "two" + -plus "fold." Verb meaning "to work as, in addition to one's regular job" is c.1920, circus slang, from performers who also played in the band. Double-header is first recorded 1869, Amer.Eng., originally a kind of fireworks or a railway train pulled by two engines; baseball sense is c.1890. Double Dutch "gibberish" is attested from 1864 (High Dutch for "incomprehensible language" is recorded from 1789). Double agent is first attested 1935; double date is from 1931. Double-take and double talk both first attested 1938. Military double time (1833) was originally 130 steps per minute; in modern U.S. Army 180 steps of 36 inches in a minute. To double park a motor vehicle is recorded from 1931.
Quickly; rapidly: When I holler, come on the double(1892+)
A person or thing that strongly or exactly resembles another; duplicate; dead ringer, look-alike: She's Grace Kelly's double(1543+)
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with on the double
Very quickly, as in You'd better get here on the double. This expression, also put as at the double, came from the military, where it means “double time”—that is, marching twice as fast as normally.
[ Early 1800s