1Having no conscience; acting or inclined to act without regard for what is right; unscrupulous. Also: failing to perform one's work or duty well or thoroughly; careless, irresponsible.
2Made, done, or gained without regard for conscience or duty; exhibiting or characterized by unscrupulousness or irresponsibility.
Now frequently in legal contexts.
Mid 17th century; earliest use found in Robert Baillie (1599–1662), Church of Scotland minister and author. From un- + conscientious.