Green’s Dictionary of Slang

1. a woman.

1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1859		Matsell Vocabulum.

2. (Irish) the ‘tail’ (reverse side) of coin; thus a halfpenny minted in Ireland (see cit. 1823).

1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: 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.
1802		M. & R. Lovell Edgeworth Essays on Irish Bulls 128: A quarrel happened between two shoeblacks, who were playing at what in England is called pitch farthing, or heads and tails, and in Ireland, head or harp.
1811		Lex. Balatronicum [as cit. 1785].
1823		‘Jon Bee’ Dict. of the Turf, the Ring, the Chase, etc. 93: Harp — the arms of Ireland, placed on the reverse of their copper coin, whereby halfpence get that denomination.
1926		S. O’Casey Plough and the Stars Act III: Go on, toss them again [...] Harps, a tanner.
1851		Lancaster Gaz. (Lancs) 28 June 3/5: Palmers are persons who visit shops under pretence of collecting harp halfpence [...] while he pretends to search for harps, he contrives to conceal as much as possible in the palm of his hand.
1870		Rochdale Obs. 24 Dec. 6/2: Sometimes [...] the ‘palmers’ indice the shop-keeper to search for ‘harp’ halfpennies or ‘lion’ shillings.
1965		L. Fleming Head or Harp 30: ‘Toss you for it, head or harp.’ ‘Harp?’ ‘Look, stupid, this side of the penny’s the head, that’s the harp.’.
1982		(con. 1930s–50s) E. Mac Thomáis Janey Mack, Me Shirt is Black 116: Up went the flyers – a head and a harp [...] The cheapest horse I ever saw wasn’t sold, he was gambled on ‘harps’ at the pitch-and-toss school in the Brickfields for ten pounds.
1994		(con. 1930s) K.C. Kearns Dublin Tenement Life 124: See, on the penny there was a head on one side and a harp on the other and they’d be tossing that. It was ‘heads you win, harps you lose’.

3. (US) an Irish person.

1900		C.L. Cullen Taking Chances 238: This Harp from Connemara.
1904		C. Connors Bowery Life [ebook] About a week after dis big harp goes into a Chinese laundry for his wash wid anudder harp named Clancy.
1912		T.A. Dorgan Daffydils 9 Oct. [synd. cartoon strip] Honus Reilly the Dutch harp and Benny Dickteen were holding an earnest conversation in the Dingbat Cafe.
1924		P. Marks Plastic Age 169: ‘He’s a harp,’ said a brother scornfully. ‘At any rate, he’s a Catholic.’.
1936		(con. 1920s) Dos Passos Big Money in USA (1966) 792: The foreman was a big loud-mouthed harp.
1938		D. Runyon ‘Bred for Battle’ in Runyon on Broadway (1954) 623: A guy [...] by the name of Shamus Mulrooney, the Fighting Harp?
1942		N. Algren Never Come Morning (1988) 110: Adamovitch. Milano. Comisky. A Polack, wallio and a harp.
1955		B. Schulberg On the Waterfront (1964) 173: You dumb harp, you must like gettin’ hit in the head.
1962		R. Dougherty Commissioner 265: He was a very dumb harp that Paddy.
1973		G.V. Higgins Digger’s Game (1981) 32: Those’re hard Harps.
1987		T. Wolfe Bonfire of the Vanities 104: He’s your basic Irish who made good, but he’s still a Harp.
1992		N. Cohn Heart of the World 9: Friggin’ Harps, straight off the friggin’ boats.