Merriam-Webster's Collegiate® Dictionary, 11th Edition
MULE
I. noun
Etymology: Middle English, from Anglo-French mul, from Latin mulus
Date: 13th century
1. a hybrid between a horse and a donkey, a self-sterile plant whether hybrid or not, a usually sterile hybrid, a very stubborn person, a machine for simultaneously drawing and twisting fiber into yarn or thread and winding it into cops, a person who smuggles or delivers illicit substances (as drugs),
II. noun
Etymology: Middle French, a kind of slipper, from Latin mulleus shoe worn by magistrates
Date: 1562
a shoe or slipper without quarter or heel strap