1. to tell a bald faced lie to someone and then brazenly deny that this was ever done
2. to give a person the frustration of moving their keys, purse, wallet, condoms, or other object and to then claim they were never moved or the person had done it themself
3. a frequent tool in the slickery dick or slickery clit's toolkit for infidelity
4. an insidious tool in the hands of unscrupulous mind-screwers and sociopaths
5. to tell a lie (consciously or unconsciously) without regard for whether or not you undermine another person's sanity/ perception of reality
It comes from the 1938 movie Gas Light.
Tommy Lee was gas lighting Elizabeth Taylor until she lost her mind. Mr. Lee should be shot.
Courtney Love leveled Kurt Cobain, not just with the multiple affairs, but by gently undermining his self respect with a little known procedure called gas lighting. He didn't even know he was a pound overweight until he was 20 pounds overweight. Poor guy.
It is a sign of the slow progress of culture that something as insidious as gas lighting can fail to be widely known, except in a slang dictionary definition, nearly three quarters of a century later than the movie by the same name.