1.  A hybrid animal; specifically, one generated between an ass and a mare. Sometimes the term is applied to the offspring of a horse and a she-ass, but that hybrid is more properly termed a hinny. See Hinny. [1913 Webster]
" Mules are much used as draught animals. They are hardy, and proverbial for stubbornness." [1913 Webster]

2.  A plant or vegetable produced by impregnating the pistil of one species with the pollen or fecundating dust of another; -- called also hybrid. [1913 Webster]

3.  A very stubborn person. [1913 Webster]

4.  A machine, used in factories, for spinning cotton, wool, etc., into yarn or thread and winding it into cops; -- called also jenny and mule-jenny. [1913 Webster]

5.  A slipper that has no fitting around the heel. [WordNet 1.5]

mule, n.
1 the offspring (usu. sterile) of a male donkey and a female horse, or (in general use) of a female donkey and a male horse (cf. HINNY(1)), used as a beast of burden.
2 a stupid or obstinate person.
3 (often attrib.) a hybrid and usu. sterile plant or animal (mule canary).
4 (in full spinning mule) a kind of spinning-machine producing yarn on spindles.

mule, n. a light shoe or slipper without a back.

mule, var. of MEWL.

N  obstinacy, obstinateness, obstinacy, tenacity, cussedness, perseverance, immovability, old school, inflexibility, obduracy, obduration, dogged resolution, resolution, ruling passion, blind side, self-will, contumacy, perversity, pervicacy, pervicacity, indocility, bigotry, intolerance, dogmatism, opiniatry, opiniativeness, fixed idea, fanaticism, zealotry, infatuation, monomania, opinionatedness opinionativeness, mule, opinionist, opinionatist, opiniator, opinator, stickler, dogmatist, bigot, zealot, enthusiast, fanatic, obstinate, tenacious, stubborn, obdurate, casehardened, inflexible, balky, immovable, unshakable, not to be moved, inert, unchangeable, inexorable, mulish, obstinate as a mule, pig-headed, dogged, sullen, sulky, unmoved, uninfluenced unaffected, willful, self-willed, perverse, resty, restive, restiff, pervicacious, wayward, refractory, unruly, heady, headstrong, entete, contumacious, crossgrained, arbitrary, dogmatic, positive, bigoted, prejudiced, creed- bound, prepossessed, infatuated, stiff-backed, stiff necked, stiff hearted, hard-mouthed, hidebound, unyielding, impervious, impracticable, inpersuasible, unpersuadable, intractable, untractable, incorrigible, deaf to advice, impervious to reason, crotchety, obstinately, non possumus, no surrender, ils n'ont rien appris ne rien oublie.

N  unconformity, nonconformity, unconformity, disconformity, unconventionality, informality, abnormity, abnormality, anomaly, anomalousness, exception, peculiarity, infraction of law, breach, of law, violation of law, violation of custom, violation of usage, infringement of law, infringement of custom, infringement of usage, teratism, eccentricity, bizarrerie, oddity, je ne sais quoi, monster, monstrosity, rarity, freak, freak of Nature, weirdo, mutant, rouser, snorter, individuality, idiosyncrasy, originality, mannerism, aberration, irregularity, variety, singularity, exemption, salvo, nonconformist, nondescript, character, original, nonesuch, nonsuch, monster, prodigy, wonder, miracle, curiosity, flying fish, black sheep, black swan, lusus naturae, rara avis, queer fish, mongrel, random breed, half-caste, half-blood, half-breed, metis, crossbreed, hybrid, mule, hinny, mulatto, tertium quid, hermaphrodite, phoenix, chimera, hydra, sphinx, minotaur, griffin, griffon, centaur, saggittary, kraken, cockatrice, wyvern, roc, dragon, sea serpent, mermaid, merman, merfolk, unicorn, Cyclops, men whose heads do grow beneath their shoulders, teratology, fish out of water, neither one thing nor another, neither fish nor fowl, neither fish flesh nor fowl nor good red herring, one in a million, one in a way, one in a thousand, outcast, outlaw, off the beaten track, oasis, uncomformable, exceptional, abnormal, abnormous, anomalous, anomalistic, out of order, out of place, out of keeping, out of tune, out of one's element, irregular, arbitrary, teratogenic, lawless, informal, aberrant, stray, wandering, wanton, peculiar, exclusive, unnatural, eccentric, egregious, out of the beaten track, off the beaten track, out of the common, out of the common run, beyond the pale of, out of the pale of, misplaced, funny, unusual, unaccustomed, uncustomary, unwonted, uncommon, rare, curious, odd, extraordinary, out of the ordinary, strange, monstrous, wonderful, unexpected, unaccountable, outre, out of the way, remarkable, noteworthy, queer, quaint, nondescript, none such, sui generis, unfashionable, fantastic, grotesque, bizarre, outlandish, exotic, tombe des nues, preternatural, denaturalized, heterogeneous, heteroclite, amorphous, mongrel, amphibious, epicene, half blood, hybrid, androgynous, androgynal, asymmetric, adelomorphous, bisexual, hermaphrodite, monoclinous, qualified, singular, unique, one-of-a-kind, newfangled, novel, non-classical, original, unconventional, unheard of, unfamiliar, undescribed, unprecedented, unparalleled, unexampled, unconformably, except, unless, save barring, beside, without, save and except, let alone, however, yet, but, once in a blue moon, once in a million years, Int, what on earth!, what in the world!, What the devil!, Holy cow!, Can you top that?, Sacre bleu, never was seen the like, never was heard the like, never was known the like, I could hardly believe it, I saw it, but I didn't believe it.

N  carrier, porter, bearer, tranter, conveyer, cargador, express, expressman, stevedore, coolie, conductor, locomotive, motor, beast, beast of burden, cattle, horse, nag, palfrey, Arab, blood horse, thoroughbred, galloway, charger, courser, racer, hunter, jument, pony, filly, colt, foal, barb, roan, jade, hack, bidet, pad, cob, tit, punch, roadster, goer, racehorse, pack horse, draft horse, cart horse, dray horse, post horse, ketch, Shetland pony, shelty, sheltie, garran, garron, jennet, genet, bayard, mare, stallion, gelding, bronco, broncho, cayuse, creature, critter, cow pony, mustang, Narraganset, waler, stud, Pegasus, Bucephalus, Rocinante, ass, donkey, jackass, mule, hinny, sumpter horse, sumpter mule, burro, cuddy, ladino, reindeer, camel, dromedary, llama, elephant, carrier pigeon, pallet, brace, cart, dolley, support, fork lift, carriage, ship, equine, asinine.

mule deer, mule driver, mule fat, mule killer, mule skinner