It's from an old film called Gaslight where a husband tries to convince his wife she is mad by, for example, hiding a precious necklace and then making her believe she must have lost it somewhere in one of her regular crazy spells.
It's actually called Gaslight because he used to go out every night and leave her alone in the house. Every night her lights would flicker and she would hear bangs and crashses, thus adding to her belief she was crazy. In fact, he was only pretending to go out, he was actually in the attic looking for her long lost family loot. The lights dimmed because he switched the gas light on in attic when he was trying to find the treasure.
The arse.