c. 1600, "one who studies and resolves cases of conscience," from French casuiste (17c.) or Spanish casuista (the French word also might be from Spanish), Italian casista, all from Latin casus (see case (n.1)) in its Medieval Latin sense "case of conscience." Often since 17c. in a sinister or contemptuous sense. Related: Casuistic; casuistical; casuistically; casuistry.
Casuistry ... destroys, by distinctions and exceptions, all morality, and effaces the essential difference between right and wrong. [Bolingbroke, 1736]
