Green’s Dictionary of Slang

to work at two jobs simultaneously.

c.1786		Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: Dragooning it. A Man who occupies two branches of one profession, as a Physician compounding his own Prescriptions.
1788, 1796		Grose Classical Dict. of the Vulgar Tongue (2nd, 3rd edn) n.p.: Dragooning it. A man who occupies two branches of one profession, is said to dragoon it; because, like the soldier of that denomination, he serves in a double capacity. Such is a physician who furnishes the medicines, and compounds his own prescriptions.
1811		Lex. Balatronicum.