To gaslight someone means driving that person
crazy using
mind tricks, building up
paranoia and causing
disorientation.
The term comes from the Oscar winning film "Gaslight" (1944) in which a husband convinces his wife she is losing her mind (for instance, he secretly plays around with the gaslight).
The movie "Gaslight" was directed by George Cukor, starring Charles Boyer and Ingrid Bergman (and a guest appearance by Angela Lansbury).