During the Yuan, weak and cowardly husbands who allowed their wives to have affairs were described as turtles with their heads pulled in. As a result, the turtle (and the tortoise too, since the Chinese don't distinguish between them) became bound up with insinuations of cuckholdry.
元代更把任妻外遇的懦弱丈夫比喻為縮頭烏龜，烏龜因此成為曖昧不明，暗示男人戴綠帽子的象徵。