mule

noun

the offspring of a donkey and a horse; esp., the offspring of a jackass and a mare: mules are nearly always sterile
a small tractor or electric engine used to tow boats along a canal or move mine cars
a machine for drawing and spinning cotton fibers into yarn and winding the yarn on spindles
a hybrid animal, as the offspring of a canary and some other finch; esp., a sterile hybrid
Informal a stubborn person
Slang a person who smuggles drugs or narcotics into a country

noun

a lounging slipper that does not cover the heel

See mule in American Heritage Dictionary 4