At a guess, it's "you mule!" said in utter exasperation.
Mules are much smarter than horses and apt to become stubbornly resistant if asked to do something they don't trust. At between 350 kg for a small mule and 500 kg for a mammoth, no human can shift a mule if the mule doesn't agree. They can also kick hard enough to break a leg, backward, diagonally and sideways.
With mules, carrots and cuddles work much better than sticks. Something we humans could learn from.
This post was edited by Benedict Arnold at June 14, 2018 5:58 PM MDT