Green’s Dictionary of Slang

coaping-cull n.

a jockey.

1753		J. Poulter Discoveries (1774) 42: I am a coaping Cull; I am a Horse Jockey.
1786		Whole Art of Thieving 31: They [...] go thirty or forty miles that night, towards the next gaff to fence them [i.e. horses], that is, to the next fair to sell them, and when they come there they sell them to a coaping cull, that is, a horse jockey they know.