From Bai Juyi's (
白居易) Tang poem 《
琵琶行》 (Song of the Pipa):
千呼万唤始出来,
犹抱琵琶半遮面 — 'Called a thousand times, ten thousand times, at last she emerged, still holding the pipa, half-hiding her face.' Describes a shy pipa-player on a Xunyang riverboat. Today mostly used figuratively for anything coyly half-disclosed.