The time of year between Late Fall and Early Winter, when it's not quite either, but a hybrid of both. Usually that time between Thanksgiving and Christmas. Most commonly, the weather seems to shift back and forth between warm and cold very frequently. Winter does not officially begin until December 21st, making the earlier part of December technically fall. But you're not really fall or winter, so you're Finter.
It snowed like mad yesterday, and then today it was 80 degrees. Certain feels like Finter weather !
It's early December and and feels kind of warm to buy a Christmas tree. It's not really, not quite yet winter. It's finter!
The Rule is an acknowledgment of one's own mortality and is invoked as a means of avoiding activities, people, movies, organizations, books and TV shows that are very likely a total waste of time.
In essence, it means that each of us only have a finite number of summers left, and those summers aren't to be frittered away on meaningless crap.