'Yī sī bù gǒu' literally means 'not even one thread carelessly' — a vivid metaphor suggesting that someone pays attention to the tiniest detail, like a single silk thread, and refuses to be careless or sloppy in any way. The idiom originates from classical Chinese texts describing strict moral discipline and meticulous craftsmanship.

It describes a person’s attitude: extremely careful, precise, principled, and conscientious — especially in work, study, or artistic creation. It often implies high personal standards and integrity, not just technical accuracy. While it can describe behavior (e.g., editing a document), it's most commonly used attributively before nouns (e.g., 一丝不苟的态度) or predicatively with 是 or 很. It carries positive connotations of reliability and excellence, though occasionally hints at rigidity if overemphasized.