Don't call someone jackleg unless you're prepared for that person to get angry with you. Throughout its more than 150-year-old history in English, jackleg has most often been used as a term of contempt and deprecation, particularly in reference to lawyers and preachers. Its form echoes that of the similar blackleg, an older term for a cheating gambler or a worker opposed to union policies. Etymologists know that blackleg appeared over a hundred years before jackleg, but they don't have any verifiable theories about the origin of either term.