anything that comes off the surface:
I.foam, froth, of the sea, Od.; of wine, Eur.; ἄχνη οὐρανία the dew of heaven, Soph.; δακρύων ἄχνη dewy tears, id=Soph.
II.of solids, the chaff that flies off in winnowing, in pl., Il.; the down on fruits, Anth.
III.ἄχνην in acc. as adv., a morsel, the least bit, Ar.