The word 刀光剑影 is formed by directly combining the two-character phrases 刀光 (knife gleam) and 剑影 (sword shadow), where each phrase pairs a noun for a weapon (刀, knife; 剑, sword) with a noun for a visual phenomenon (光, light; 影, shadow). This juxtaposition creates a literal depiction of the gleams and shadows of blades, resulting in a four-character idiom that concretely describes the visible aspects of a combat scenario.