"Fen" and "faans" were both slang terms for fans. The former was more widely used in the old-school fan community, while the later had the advantage of being immediately clear to people not in the fan community and the disadvantage of not really being used by anyone by the time the series came out. I should have made it FANS from the start.
Tim's obscene gesture here is the result of a power struggle between me and Jason. I wanted a middle finger and a "Fuck the establishment," Jason had him raise his fist instead, so I changed Tim's line to give it an even more obscene spin. It's funny; there were "Tims" in my Science Fiction Club, but I didn't realize I had an internal Tim until then.