The verb "to gaslight" is named after a lamp, by way of a play.
Gaslight the noun is first recorded in 1808, according to Etymonline, for light from burning coal gas or a jet that provides it. It lit streets and homes through the 19th century and into the early 20th.
The verb comes from fiction. Patrick Hamilton's 1938 British stage play Gas Light centers on a husband who tries to convince his wife that she is losing her sanity. Britannica says he manipulates their surroundings, such as dimming the gaslights, and denies what she perceives, which gradually erodes her confidence in her own judgment. The play was a significant success.
Two film adaptations followed, called Gaslight, released in the United Kingdom in 1940 and in the United States in 1944. In the plot, as Britannica describes it, the husband convinces his wife that she is a kleptomaniac and that she imagines the sounds in the attic and the dimming lights. He is actually causing both while searching for her aunt's missing jewels. Etymonline adds that a family friend notices the lights dimming too, which breaks the spell.
The verb took time. Merriam-Webster gives 1953 as the first known use of the verb in the sense of psychologically manipulating someone. Etymonline dates it to about 1956, with firm attestation by 1961. The sources do not agree on the first year.
It gained force in feminist writing in the late 1970s. Etymonline notes a 1980 work by Florence Rush that used it for efforts to undermine another person's perception of reality. By 2016, it had also come to mean dismissing or discrediting someone's viewpoint.
Merriam-Webster chose "gaslighting" as its 2022 Word of the Year, based on lookups that rose 1,740% over the year. The lamp in the play has faded from daily life, but the title now names the tactic.