G'day Ebbie,

I seem to remember reading that "donkey" was introduced as a euphemism for "ass" ... which, in areas prone to pronouncing 'a'in a longer form was being pronounced - and confused with - one's fundament. "Donkey", apparently means something like "small, dark brown" creature.

That said, the Oxford Dictionary lot reckon it's just the familiar form of the man's name Duncan ... but it's all too far back for good historical evidence!

Regard(les)s,

Bob Bolton