Definition of "blackleg"

[H]ere, then, was a community of good taste and kind feeling, no sharpers, no black-legs, no wolves in sheep's clothing.

1836 December 31, Laurie Todd, “Letter from Laurie Todd: Christmas and New-Year’s-Day”, in New-York Mirror, a Weekly Journal, Devoted to Literature and the Fine Arts, volume XIV, number 27, New York, N.Y.: Scott & Co., printers, page 211, column 1

[Y]our friend Bloundell-Bloundell is a professional blackleg, and travels the Continent, where he picks up young gentlemen of fashion and fleeces them.

1848 November – 1850 December, William Makepeace Thackeray, chapter 32, in The History of Pendennis. […], volumes (please specify |volume=I or II), London: Bradbury and Evans, […], published 1849–1850

They have no regard for respectability like yours — nothing between a baron and a blackleg.

1852?, The Sporting Magazine (page 185)

I had never defrauded a man of a farthing, nor called him knave behind his back. But now the last rag that covered my nakedness had been torn from me. I was branded a blackleg, card-sharper, and murderer.

1908, W[illiam] B[lair] M[orton] Ferguson, chapter II, in Zollenstein, New York, N.Y.: D. Appleton & Company, page 25

In the autumn there was a row at some cement works about the unskilled labour men. A union had just been started for them and all but a few joined. One of these blacklegs was laid for by a picket and knocked out of time.

1918, W[illiam] B[abington] Maxwell, chapter XLIV, in The Mirror and the Lamp, Indianapolis, Ind.: The Bobbs-Merrill Company, pages 364–365

It's in the evening after dark when the blackleg miner creeps to work. With his moleskin pants and his dirty shirt, there goes the blackleg miner.

1970 June, traditional (lyrics and music), “The Blackleg Miner” (track 4), in Hark! The Village Wait‎, performed by Steeleye Span

Why was I there, munitioning, blacklegging, slaving as though my bread depended on it?

1939, Philip George Chadwick, The Death Guard, page 154