If you want my best guess then it would be "flash stop".
In early photography there wasn't as much concern with DOF as there was simply getting a good, clear image. They didn't have the luxury of playing with light for DOF's sake... because simply getting adequate exposure was more the challenge... and so the camera iris had to stay open for rediculous amounts of time. As cams improved and the flash became common, early photographers suddenly found themselves in the situation of having to consider aperature as much as shutter time... and my guess is that's when the term started as it related to use of a flash.
So approximating the distance at which the flash would no longer be effective, or "stop" most likely is the source of the term f-stop.
I have no idea if this is correct by the way... this is just what I think would be the most logical explanation.