According to the fvwm FAQ, no one knows what the "f" in fvwm stands for, not even the original author, Rob Nation (who says he's forgotten). They suggest you make up your own word. I think of mine as a "functional virtual window manager": It gets the job done without a lot of bells and whistles. The official website suggests any of the following:
Feeble, Fabulous, Famous, Fast, Foobar, Fantastic, Flexible,
F!@#$%, Flashy, FVWM (the GNU recursive approach), Free, Final,
Funky, Fred's (who the heck is Fred?), Freakin', Flawed,
Father-of-all, Feivel (the mouse from "An American Tail"), etc.