Why is Robert in Downton Abbey called as Donk?

In "Downton Abbey," Robert Crawley, the Earl of Grantham, is affectionately called "Donk" by his family and close friends. The nickname is derived from "donkey," which is a playful reference to his sometimes stubborn nature and his tendency to be a bit set in his ways. This endearing term reflects the warmth and familiarity within the Crawley family, highlighting their close relationships despite the formalities of their social status.