1To administer poison to (a person or animal); to kill or injure with poison; = "poison". Also without object.
2To add poison to (food, drink, etc.); to cover or impregnate (an object) with poison so as to make it harmful; to make poisonous or venomous.
3To pollute or adulterate with a harmful or noxious substance; to contaminate.
4To embitter (a person, his or her feelings, a relationship, etc.); to make hostile or malignant; to turn (a person or his or her mind) against.
Late Middle English; earliest use found in William of Palerne. From Anglo-Norman empoisoner, enpoisoner and Middle French empoisonner, enpoisonner (French empoisonner) to kill with poison, to make (something) harmful with poison from em- + poison.