Blackleg

a rascal, swindler, or card cheat. The derivation of this term was solemnly argued before the full Court of Queen's Bench upon a motion for a new trial for libel, but was not decided by the learned tribunal. Probably it is from the custom of sporting and turf men wearing black top-boots. Hence blackleg  came to be the phrase for a professional sporting man, and thence for a professional sporting cheat. The word is now in its worst sense diminished to “leg.”