In a fast manner; quickly and swiftly (archaic or non-standard form; 'quickly' is preferred in modern English).
From 'fast' (the adjective meaning quick) plus '-ly,' the standard adverbial suffix; however, 'fast' itself works as an adverb, making this form archaic or dialectal.
Grammarians hate 'fastly' because 'fast' already works perfectly as an adverb (you say 'go fast,' not 'go quickly')—it's a word that shows how English speakers sometimes add '-ly' to words that don't need it!
Complete word intelligence in one call. Free tier — 50 lookups/day.