The term comes from the 1944 film Gaslight, in which a husband dims the gas lights and denies it to make his wife doubt her sanity. The serious meaning — sustained manipulation that makes a victim distrust their own perceptions — remains important in relationship and mental-health content.
Internet usage diluted it: people now say "stop gaslighting me" about a friend insisting a restaurant is good, or invite positive gaslighting ("gaslight me into believing I love running"). The "gaslight gatekeep girlboss" meme (a parody of girlboss-era feminism) pushed the word into full ironic circulation. Critics note the dilution makes it harder to name actual abuse, which is itself a recurring discourse cycle.