To take a false oath; also to curse. He rapped out a volley; i.e. he swore a whole volley of oaths. To rap, means also to exchange or barter: a rap is likewise an Irish halfpenny. Rap on the knuckles; a reprimand.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Rantum Scantum * RappareesNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes