Comes from the 'Book of Han' (《
汉书·
终军传》): in the Western Han, the young envoy
终军 (Zhōng Jūn) requested 'a long rope' (
长缨, cháng yīng) to bind the king of Nanyue and bring him back to court —
请缨 literally means 'to request a rope.' Two thousand years on, it's still the standard way to say 'voluntarily ask for a tough mission,' especially in patriotic and emergency-response contexts.