Emily Dickinson Lexicon

FAST, a.² [W. fêst, fast, quick; festu, to hasten; L. festino. If f is not written for h, as in haste, see Class Bz, No. 44, 45, 46, the sense is to press, drive, urge, and it may be from the same root as the preceding word, with a different application.]

Swift; moving rapidly; quick in motion; as, a fast horse.