The word "一丝不苟" is formed by combining four characters: 一 (one), 丝 (silk thread), 不 (not), and 苟 (careless). Literally, it means "not one thread is careless", where the silk thread represents a minute detail. This literal image metaphorically constructs the idiom's meaning of being thorough and meticulous, as neglecting even a single thread would imply carelessness.