Probably akin to the base of akmen ("acid"); keen; by analogy, rapid -- sharp, swift.
see GREEK akmen
1. sharp (from Homer down): ῤομφαία, δρέπανον, Revelation 1:16; Revelation 2:12; Revelation 14:14, 17; Revelation 19:15 (Isaiah 5:28; Psalm 56 2. swift, quick (so from Herodotus 5, 9 down; cf. ὠκύς fleet): Romans 3:15 (Amos 2:15; Proverbs 22:29).