In Roman military terminology, a line of battle or array of soldiers; also an edge or sharp point.
From Latin 'acies,' originally meaning 'sharpness' or 'edge,' then extended to mean 'battle line' because armies were arranged in sharp, pointed formations. The word carries its military meaning into English.
Roman generals obsessed over acies formations—a shield wall arranged wrong could collapse, so they practiced 'acies' drills until soldiers could hold formation even while surrounded!
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