The word **毫发 (háofà)** is a classical Chinese **binomial compound** formed by combining two near-synonymous characters, each referring to extremely fine and small bodily hairs. **毫 (háo)** originally meant the fine, downy hair on an animal or the tip of a brush, while **发 (fà)** specifically means human head hair. When paired, they create a powerful **metaphorical idiom** that signifies the tiniest, most minute detail or quantity—essentially "the smallest hair." This synergy emphasizes precision and minuteness, leading to its modern figurative meaning of "the slightest bit" or "a hair's breadth," often used in negated contexts like "毫发无损" (not harmed in the slightest).