The term comes from a 1938 play, Gas Light, in which a husband quietly dims the gas lamps and then insists his wife is imagining it.
The mechanism is cumulative rather than a single event. "I never said that." "You're exaggerating." "You're too sensitive." "You're imagining things." Each, taken alone, is an ordinary line from an argument. Repeated systematically, they produce a specific effect: you start checking your own memories against the other person instead of relying on them.
The signature isn't the content of the lines but the result: you begin keeping evidence. Saved messages, notes about conversations, witnesses. Needing to document your own reality is the clearest indicator that something is wrong.