Sentence:  I had squeezed the trigger of my own gun , and to my amazement , it had fired and kicked back into my shoulder with the force of an angry <WSD>mule</WSD> ; and then I was adding my own voice to the crescendo of sound , hurling more vile language than I ever thought I knew , sobbing and shouting , and aware that if I had passed water before , it was not enough , for my pants were soaking wet . 
Meaning of the word marked with <WSD> tags:  hybrid offspring of a male donkey and a female horse; usually sterile