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).