Fast is a curious word indeed. Historically speaking, it definitely started life as an adverb, but probably not the one about which you're thinking! Proto-Germanic *fasto was used in the sense of "Tie the boat fastto the dock!", where fast means "securely". Its use as an adjective ("I drive a fast car") was first observed around 1550.
Fast meaning quickly had actually been attested as early as 1200! But curiously, English prescriptive grammarians have railed against that usage for centuries. "Don't say 'fast'; say 'quickly'!" is one of the oldest of the prescriptive grammarians' harangues.
1. Can you think of any other "dual purpose" adjective/adverbs in English?
2. Do you think English should accept "fast" as an adverb? Why or why not?