Life-guard men, from their sitting on horseback, under an arch, where they are frequently observed to drive away flies with their swords.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Fly-by-night * FlyerNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes