The word "光明正大" is formed by combining two two-character compounds: "光明", meaning bright and open, and "正大", meaning upright and great. Each character contributes directly: 光 (light) and 明 (bright) together suggest transparency, while 正 (correct) and 大 (great) together suggest righteousness, resulting in an idiom that denotes honorable and just conduct through the combined meanings of its components.