vicious adj.

late 14c., "unwholesome, impure, of the nature of vice, wicked, corrupting, pernicious, harmful;" of a text, "erroneous, corrupt," from Anglo-French vicious, Old French vicios "wicked, cunning, underhand; defective, illegal" (Modern French vicieux), from Latin vitiosus (Medieval Latin vicious) "faulty, full of faults, defective, corrupt; wicked, depraved," from vitium "fault" (see vice (n.1)).

Meaning "inclined to be savage or dangerous" is first recorded 1711 (originally of animals, especially horses); that of "full of spite, bitter, severe" is from 1825. In law, "marred by some inherent fault" (late 14c.), hence also this sense in logic (c. 1600), as in vicious circle in reasoning (c. 1792, Latin circulus vitiosus), which was given a general sense of "a situation in which action and reaction intensify one another" by 1839. Related: Viciously (mid-14c., "sinfully"); viciousness.