Short for "fifty I'd like to fuck." It refers to an attractive woman who is fifty or in her fifties, but still too young to be considered a Gilf. A Filf is typically someone you find sexually appealing in that age range. While the term "milf" is commonly used for women in their 30s or 40s, a Filf specifically denotes an attractive older woman in her fifties who catches one's eye.
Example: "Did you see that woman at the gym? Total Filf material."
variations on the ever-imposing MILF we all know.
filf is a female i'd like to fuck, dilf is a dick i'd like to fuck; something a girl can say about any guy she can't get her ugly hands on , but really wants to fuck badly.
aha, gotcha!... katie you're not callin' me again your dilf ! see, i thought i was older than your mom.