Gaslighting is when someone arguing with you lies and twists things so convincingly that you wonder, "Am I crazy? Am I losing my memory?" If someone takes your brownie and says, "No, you ate it, remember? You even said you liked the nuts in it!" — that’s gaslighting.
This sense of gaslighting first appeared in the 1960s and is now the only meaning, since gas has not been used for lighting for a long time. It comes from a British play called Gas Light, written in 1838 by Patrick Hamilton. The play was a thriller in which a man causes his wife to question her sanity by secretly changing the brightness of the gas lights in their home when leaving her alone, all the while pretending that no such change has occurred.