1
general::
Serve in two capacities, as in In this company everyone is asked to double in brass occasionally. This expression was originally used in the traveling circus, where, for example, a clown was Also expected to play an instrument in the street parade. The original allusion, however, is to an instrumentalist who plays more than one instrument in an ensemble, a practice particularly common among players of brass instruments. [Late 1800s]
American Heritage Idioms
2
general::
Phrase(s): double in brass (as something)
Fig. to serve in two capacities. (Alludes to a musician who could play the brass instruments as well as other instruments.) • Wally was our bookkeeper and doubled in brass as a clerk.
McGrawhill's American Idioms And Phrasal Verbs