An Etymological Dictionary of the German Language/Annotated/fast
fast, adverb, ‘almost, nearly,’ from Middle High German vaste, vast, adverb (from vęste, ‘firm’), ‘firmly, strongly, powerfully, very, very quickly,’ Old High German vasto, adverb, from fęsti; similar unmutated adverbs from mutated adjectives are schon from schön, spat from spät. Modern High German has also turned fest into an adverb, the older adverb fast having been specialised in meaning; even in Middle High German vęste is an adverb.