jack·leg 
(jăklĕg′) Chiefly Southern & South Midland US
(jăklĕg′) Chiefly Southern & South Midland US adj.
1. Lacking skill or training; incompetent. Used especially of lawyers or preachers.
2. Unscrupulous or dishonest.
3. Makeshift; temporary.
n.
1. An unskilled or unscrupulous itinerant worker.
2. A strikebreaker.
[Perhaps JACK + (BLACK)LEG.]