The Chinese idiom "杞人忧天" is formed by directly combining four characters: 杞 (Qi, an ancient state), 人 (person), 忧 (to worry), and 天 (sky). It originates from a specific fable about a man from Qi who irrationally feared the sky would collapse, so the phrase literally means "the man from Qi worries about the sky" and structurally encodes this narrative to convey the concept of baseless anxiety.