adj

• (US) Amateur, untrained; incompetent.
  "1841, Letter to the editor, The Southern Planter, Volume I, No. 1, January 1841, p. 12, The next year I had a projecting kind of jack leg carpenter, from Hanover, living with me in the capacity of overseer […]"
• (US) Dishonest, unscrupulous.
  "The little nosy reporter with the hair was fair crazy to come, but McGinty gets a jackleg doctor to examine him an’ swear that he’s sufferin’ from spatulation o’ the medulla oblongata, housemaid’s knee, and the hives."
• (US) Ineptly built or operated; makeshift.
  "The train is late […]. ¶ “Well, if I come to such a place as this I must expect a jackleg railroad […].”"

noun

• (US) An amateur; an untrained or incompetent person.
  "[…] I, gentlemen and ladies, are a rale Scientificky! I ain’t none of your jack-legs."
• (US) A shyster or con artist; a gambler who cheats; a generally dishonest or reprehensible person.
  "“[…] we ain’t called on to believe every smooth-talking jackleg who wanders in here.”"