The word "gaslighting" comes from a play called "Gas Light." written in 1938 by Patrick Hamilton.
In this play, a husband tries to make his wife think she is losing her mind. He does some tricky things, like secretly making the lights from gas lamps in their house dimmer, and then he tells her that she's just imagining these changes. This play later became a movie.
Nowadays, when we say "gaslighting," we mean when someone tries to make another person unsure about their own memory or feelings, similar to what the husband did to his wife in the story.
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