OMG, none of you know this?
They are called donks (Donkey), because they are stubborn - they refuse to fold when all reason and logic says "you are beat, give it up". Donkey's, as in the animal, are known to be stubborn, so there you go.
As a funny aside, this in a way only makes half-sense. Stubbornness is actually a sign of intelligence. A horse, for example, is way more stupid than a mule or a donkey. They will continue to ride full gallop no matter what, which is why you have to stop them and make them rest. Otherwise, they will ride full speed until they drop dead. At the grand canyon, they have mules take you down the narrow path, not horses. Not because they don't fit, but because if they get startled, they might jump right off the edge! See, stupid horse, smart mule. Don't believe that scene in the movie "True Lies" where Arnold tries to get the horse to jump from one building to the next and the Horse stops...In reality that son-of-a-gun would have jumped, even knowing it could not make it. Back in the day of carnivals, they would lift a horse 3 story's up to jump off a board into a small pool of water - they tried a mule, and the damn thing wouldn't budge - do you blame him?!? Again, if you try to take a mule down a wash and it will not go, it's because he knows it's too dangerous.
So, they are stubborn, just like some players. Just do not confuse them with a calling station. That's a whole different beast altogether.