1A person or activity that corrupts or undermines the morals or moral principles of a person, society, etc. Now somewhat archaic (chiefly US).
2A cause of a loss of confidence, hope, or morale; something which is demoralizing.
Late 18th century; earliest use found in Tom Paine (1737–1809), author and revolutionary. From demoralize + -er Compare French démoraliseur.