There's an interesting ambiguity, in America at least, behind what we mean when we call insult someone for being an "ass." Famously, a Dickens character says "the law is a ass--an idiot." What sort of an ass is the basis of the character's metaphor? It is most likely the four-legged beast of burden, not the sitting-end of a person. Dickens, in a crude mood, would probably have called his nether-end his "arse": this is the British word for the body part because, until the nineteenth century, "ass" was reserved just for the animal. However, for me the situation is different. As a modern, urban American I don't have much experience around the animal, the "ass." An ass is still a stupid person, but the metaphor is anatomical: When I use the word I most likely have the image of the buttocks of a person (or some animal--think how "horse's ass" = "fool") when I call you an "ass"--it's the end without the brains in it, for sure.
In the nineteenth century (according to spellings in Bryan Garner's Dictionary of Modern American Usage, to the Oxford English Dictionary, and to Jonathan Lighter's Historical Dictionary of American Slang), in Britain and America "arse" began to be pronounced as what would sound to an American like "ahss" or "ass." The second edition of the Oxford English Dictionary says that "arse" became "ass" as a "vulgar and dialect sp[elling] and pronunciation of arse"; other dictionaries aren't too helpful as to the source of the change; certainly the British change at that time of most syllable-end rs to the schwa sound (the vowel in "good") must have had something to do with it.
Etymologically, "ass" the animal and "arse" the body part are very different words. The word for the animal in Old English was "assa/asse," (masc./fem.) with the alternative "esol" (see also the supplement; Germans still say "Esel"). There's a forest of cognates in the Germanic and Celtic languages that are dealt with in good etymological dictionaries like the OED, C. T. Onions' Oxford Dictionary of English Etymology, and the Chambers Dictionary of Etymology; they trace the word all the way back to a Sumerian (!) cognate anšu, with the Latin "asinus" as the direct source.
I'm of course far more interested in where "arse" comes from. It's Old English, and the standard Old English dictionary Bosworth-Toller supplies the spelling "ears" and various delightful compounds like "ears-ende" for buttocks and "ears-gang" for latrines and their contents. The Middle English Dictionary has a lengthy and fascinating entry on "ars." The other older Germanic languages had cognates and there's a Greek cognate ὄρσος which evolved into ὄρρος; Eric Partridge's Origins points to a Hittite cognate arraš. There's apparently a proto-Indo-European cognate as well.
OK. So what's the difference between an ass and a donkey? Nothing, apparently (I think Americans, in view of the anatomical "ass," will always say "donkey"); it's a dialect or slang word first recorded in 1785 in George Grose's Classical Dictionary of the Vulgar Tongue.
In the nineteenth century (according to spellings in Bryan Garner's Dictionary of Modern American Usage, to the Oxford English Dictionary, and to Jonathan Lighter's Historical Dictionary of American Slang), in Britain and America "arse" began to be pronounced as what would sound to an American like "ahss" or "ass." The second edition of the Oxford English Dictionary says that "arse" became "ass" as a "vulgar and dialect sp[elling] and pronunciation of arse"; other dictionaries aren't too helpful as to the source of the change; certainly the British change at that time of most syllable-end rs to the schwa sound (the vowel in "good") must have had something to do with it.
Etymologically, "ass" the animal and "arse" the body part are very different words. The word for the animal in Old English was "assa/asse," (masc./fem.) with the alternative "esol" (see also the supplement; Germans still say "Esel"). There's a forest of cognates in the Germanic and Celtic languages that are dealt with in good etymological dictionaries like the OED, C. T. Onions' Oxford Dictionary of English Etymology, and the Chambers Dictionary of Etymology; they trace the word all the way back to a Sumerian (!) cognate anšu, with the Latin "asinus" as the direct source.
I'm of course far more interested in where "arse" comes from. It's Old English, and the standard Old English dictionary Bosworth-Toller supplies the spelling "ears" and various delightful compounds like "ears-ende" for buttocks and "ears-gang" for latrines and their contents. The Middle English Dictionary has a lengthy and fascinating entry on "ars." The other older Germanic languages had cognates and there's a Greek cognate ὄρσος which evolved into ὄρρος; Eric Partridge's Origins points to a Hittite cognate arraš. There's apparently a proto-Indo-European cognate as well.
OK. So what's the difference between an ass and a donkey? Nothing, apparently (I think Americans, in view of the anatomical "ass," will always say "donkey"); it's a dialect or slang word first recorded in 1785 in George Grose's Classical Dictionary of the Vulgar Tongue.