The watch-word among highwaymen, signifying the person is a friend, and must pass unmolested. Music is also an Irish term, in tossing up, to express the harp side, or reverse, of a farthing or halfpenny, opposed to the head.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Mushroom * MuteNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes