In pre-modern European mythology, an infant that was secretly exchanged for a mother's own baby by an evil creature. (In British, Irish and Scandinavian mythology the exchanged infants were thought to be those of fairies, sprites or trolls; in other places, they were ascribed to witches, devils, or demons.)
User Defined Definitions
"a person of subnormal intelligence"
"a child secretly exchanged for another in infancy"