There was a movie in, I think, 1944, or 1937, and it was called "Gas Light", it was based on a play in London, and another movie, but the remake from 1944 with Charles Boyer, and Ingrid Bergman is the one everyone knows, and where the term comes from. In the movie, a man tries to drive his new bride insane, in their London brownstone, by slowly convincing her she as lost her grip on her sanity. He steals things he gives her, and convinces her she lost them. He hides things and just generally wages psychological warfare on here, to break her down. The actual "gaslights" are old fashioned lamps, and the movie features one prominently, that she keeps insisting is on, but no one will believe her, even though it very much is, during the middle of the night. The movie is considered a classic, and worth watching, if you like noir/suspense.
The term is used mostly in relationships, now, vis a resurgence on instagram and such, where often (men) "gaslight" their spouse, "you embarrassed me", or "you are always flirting with my friends", "you made me look stupid", so the persons self worth and self value plummets, and the gaslighter assumes more and more control of the relationship, and the spouse questions less and less. In the movie, Boyer, needs to gain control of the estate, by committing Bergman to an insane asylum, so he can find the jewels he hid there years earlier. Also, re: infidelity, people discuss this commonly, like if you are cheating, accuse the other person.
In politics, it is the Fox News/GOP spin on any facts that make the admin look bad.
