To harp upon; to dwell upon a subject. Have among you, my blind harpers; an expression used in throwing or shooting at random among the crowd. Harp is also the Irish expression for woman, or tail, used in tossing up in Ireland: from Hibernia, being represented with a harp on the reverse of the copper coins of that country; for which it is, in hoisting the copper, i.e. tossing up, sometimes likewise called music.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Harmans * HarridanNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes