It’s like a happening but it refers to the situation where hard core bitcoin believers loose their faith and switch their bitcoins to another crypto, most probably to Ethereum.
This excellent summary by ‘Hackernoon’ set out the background to the possible flippening event and they state that :
‘There will be a time in the near future where Bitcoin is no longer the dominant “coin” or blockchain, this will likely happen sooner than most expect… and Ethereum will be the one to take its place.’
Watch this space!