The term “Flappening” is actually a colloquial term that was coined back in 2018 by Charlie Lee. The term pertains to the moment when Litecoin (LTC) exceeded the Bitcoin Cash (BCH) with consideration to market capitalization.
It is actually a play on the words on “Flippening”, another term referring to a moment within digital coin history wherein Ethereum is seen to potentially overtake the value of its forerunner Bitcoin (BTC), again, with its market cap.
However, unlike Flippening that only happened in April 2019, the Flappening had already taken place with LTC’s market cap exceeding that of BCH back in December 14th 2018. The term had since expanded to refer to the overtaking of other important altcoins.