The Collaborative International Dictionary
Acinaces \A*cin"a*ces\, n. [L., from Gr. ?.] (Anc. Hist.) A short sword or saber.
n. (context historical ancient history English) A short sword or saber.
The acinaces, also spelled akinakes ( Greek ἀκῑνάκης) or akinaka (unattested Old Persian *akīnaka, Buddhist Sogdian kynʼk) is a type of dagger or short sword used mainly in the first millennium BC in the eastern Mediterranean region, especially by the Medes, Scythians and Persians, then by the Greeks.
The acinaces is of Scythian origin, but was made famous by the Persians, and rapidly spread throughout the ancient world. The Romans believed this weapon originated with the Medes.
The acinaces is typically 35–45 cm. (14-18 in.) in length and double-edged, and although there is no universal design, the guard may be lobed with the hilt resembling that of a bollock dagger, or the pommel may be split or of the "antenna" type. The scabbard as much as anything else defines the acinaces and usually has a large decorative mount near the opening allowing it to be suspended from a belt on the wearer's right side.
Since the acinaces seems to have been a thrusting weapon, and since it was typically worn on the right, it was likely intended to be suddenly drawn with the blade facing down for surprise stabbing attacks.