“flip” is like, when someone dies or is executed and is revealed to be a certain role or alignment – e.g. if I say “katze flipped mafia” I mean “katze died and was revealed to be mafia”
a “preflip” is when you make a read based on the assumption that another player is a specific alignment, even though they haven’t flipped yet – e.g. if I say “katze is obviously a wolf, so Chloe is scum for defending them” I am preflipping katze as mafia/evil to accuse Chloe of also being evil