What does "donkey" mean?

Donkey: A stupid or obstinate fellow; an ass. Donkey engine, a small auxiliary engine not used for propelling, but for pumping water into the boilers, raising heavy weights, and like purposes. -- Donkey pump, a steam pump for feeding boilers, extinguishing fire, etc.; -- usually an auxiliary. -- Donkey's eye (Bot.), the large round seed of the Mucuna pruriens, a tropical leguminous plant.