One school of thought has the name derived from a British Signal Corps boxer, Jimmy Emblem, who was an Army Champion in 1924 and represented the Corps from 1921 to 1929.
http://2sig.kraftysworld.co.uk/other/jimmy.htm
In the Canadian Army, a signaller is often referred to as a "Jimmy" in reference to the picture of Mercury (Greek: Hermes), the roman messenger of the gods, which is referred to as Jimmy. The most widely accepted theory of where the name Jimmy comes from is a Royal Signals boxer, called Jimmy Emblem, who was the British Army Champion in 1924 and represented the Royal Corps of Signals from 1921 to 1924.
http://en.wikipedia.org/wiki/Signaller
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