1) very quickly
Part of Speech: adjective
Neuter plural of the superlative of G5036 (as adverb); most quickly, that is, (with G5613 prefixed) as soon as possible: - + with all speed.
Perseus Digital Library
University of Chicago's Logion lexicon
Thayer's dictionary plus other information.
From this site's dictionary (in Italian)
Click on the first column to search for that word as a form of the root ταχιστα; click on the second column to search for that grammatical form of the root ταχιστα; click on the third column to search for that word and grammatical form; click on the total to search for the root ταχιστα.