Varro: War’s etymologies
From On The Latin Language
Translated by Roland G. Kent
Gladium ‘sword,’ from clades ‘slaughter,’ with change of C to G, because the gladium is made for a slaughter of the enemy; likewise from its omen was said pilum, by which the enemy periret ‘might perish,’ as though perilum.
The third gate is the Janual Gate, named from Janus, and therefore a statue of Janus was set up there, and the binding practice was instituted by Pompilius, as Piso writes in his Annals, that the gate should always be open except when there was no war anywhere. The story that has come down to us is that it was closed when Pompilius was king, and afterwards when Titus Manlius was consul, at the end of the first war with Carthage, and then opened again in the same year.
The enemy are called perduelles ‘foes’; as perfecit ‘accomplished’ is formed from per ‘through, thoroughly’ and fecit ‘did,’ so perduellis is formed from per and duellum ‘war’: this word afterward became belbim. From the same reason, Duellona ‘Goddess of War’ became Bellona.
In The Story of the Helmet-Horn is the verse”:
Who for ten years fought for wages (latrocinatus) for the King Demetrius.
Those were called latrones ‘mercenaries’ from latus ‘side,’ who were at the King’s side and had a sword at their own side (afterwards they called them stipatores ‘body-guards’ from stipatio ‘close attendance’) and were hired for pay: for this pay is in Greek called λάτρον. From this, the old poets sometimes call regular soldiers latrones. But now the name latrones is given to the highwaymen who block the roads, because like regular soldiers they have swords, or else because they latent ‘lie in hiding’ to ambush their victims.