The "open-arse" (also: enter "open-ærs") entry's first reference in the NED is: "c. 1000 AElfric Gloss. in Wr.-Wülker 137/36 Mespila1/1a, openaers." This source contains no context as this is a lexicon geared at scholars (see document intro.). A note showcases the reaction of Wr.-Wülker to the word, much later, in 1884:
It is rather singular that we should find this not very delicate name
for the medlar at so early a period. It is found in the MSS.[manuscripts] of the
fifteenth century and is a word sufficiently familiar to the readers
of the popular literature of the sixteenth and seventeenth centuries.
[ Archbishop Alfric's Vocabulary of the tenth century, 137/36, note
5 ]
The NED (1888) mentions for "arse, sb.", "Obs. in polite use.". This should mean it was not always impolite. The first meaning listed refers to this being about an animal (also, ex. of the figurative meaning: "In Cheshire the stalk-end of a potato [is called] the 'arse-end of a 'tater'".) And the entry also refers to AElfric 44/2 where indeed you have Nates (Latin for buttocks), to "ears-lyre"(OE); but there is no note from the author this time over (maybe because it's not spelled "arse"? yet open-ears to mean open-arse might have been). Ass in its slang version as a dialectal variation of arse is recent (1860, popular 1930s). Considering this, a reaction seems unwarranted imho: it's about something descriptive as in the context of farming/hunting; I cannot find any alternative to Mespila or open-ærs/arse/ears in the material presented.
Blet comes from the French blettir (from blet adj., which is from an older form of blesser, but related to bruising as opposed to wounding: "Mil. XIes. blecier « meurtrir (des olives, des fruits, pour les faire mûrir) » (Raschi, Gloses dans Levy Trésor)" - as in making ripe olives, fruits by this action of bruising.) Something we casually do with our hands when some fruit is not ripe enough. Or we just wait for it to happen, as in this case systematically with some prior peeling. This answer presents the "special alteration" coined by M. Lindley for which this "blet" now stands for in context.2 Rot has a different etymology with for instance a German cognate rößen "to steep flax" ("letting raw flax steep under water for several days to break down the lignine in the stems[...]"). It may have no impact on contemporary use, but the first entry in the NED for the verb to rot relates to "animal substances" in the context of natural decomposition. In comparison blet is mostly a special form of decay for fruits.
1. More generally, the category to which the medlar belongs is interesting because other members enjoy in some cases a very rich naming tradition; such is the case with the amalanchier (also): shadbush, shadwood or shadblow, serviceberry or sarvisberry, or just sarvis, wild pear, juneberry, saskatoon, sugarplum or wild-plum, and chuckley pear. The fruits can be 50% smaller, and sometimes more of the berry type but they're related.
1a. See Plinius Natural History (AD 77–79) with translation. The TLFi etymology entry of the French word for the fruit (nèfle) refers to Plinius possibly meaning "white thorn" from the Greek "μεσπι ́ λη". This brings meaning to the medlar wikipedia entry's reference: "When the genus Mespilus is included in the genus Crataegus, the correct name for this species is Crataegus germanica Kuntze." The Crataegus is basically hawthorn/whitethorn etc. which are all, like the medlar, part of the rose family; the hawthorn is graft compatible with the medlar. In that context the haw refers to the fruit; often compared with the medlar.
2. The reference to the "special alteration" is from the same work; see NED entry, which also has: "The decomposition... of the pericarp[the "flesh", see this] begins with fermentation, and, after having passed through the intermediate stage of bletting[...], ends in the total obliteration of the cellular structure." (1864, Reader, 21 May 663). See also following entry with blet sb. where there is indication that there are no external changes appearing with sleepiness i.e. blet, as opposed to spots etc.
From the Online Etymology Dictionary:
ass (n.2)
slang for "backside," first attested 1860 in nautical slang, in popular use from 1930; chiefly U.S.; from dialectal variant pronunciation of arse (q.v.). The loss of -r- before -s- attested in several other words (such as burst/bust, curse/cuss, horse/hoss, barse/bass, garsh/gash). Indirect evidence of the change from arse to ass can be traced to 1785 (in euphemistic avoidance of ass "donkey" by polite speakers) and perhaps to Shakespeare, if Nick Bottom transformed into a donkey in "A Midsummer Night's Dream" (1594) is the word-play some think it is. Meaning "woman regarded as a sexual object" is from 1942. To have (one's) head up (one's) ass "not know what one is doing" is attested by 1969. Colloquial (one's) ass "one's self, one's person" attested by 1958.
It appears that the consonant was lost due to dialectal pronunciation of the word that eventually became popluar. Hope this helps!
Best Answer
There are some reasons to believe that this peculiarity of today's English can be ascribed to the influence of Celtic Languages with which English has been in contact for the last 1500 years.
Celtic influence
Here are a few examples:
General Celtic Influence
Several linguists including J R R Tolkien have claimed that there is a British Isles Sprachbund incorporating English and the Celtic languages. For instance the “I am working”, “I was working” is common in Welsh and Irish, but you can’t say “Ich bin arbeitend” in German.
See the book "The Celtic Roots of English" for more of these.
The Old English way of asking "what's you name" was Germanic
If you had a kind of Rosetta Stone handbook to learn Old English what would you find? Well there are actually a few ones on the net. Here is one. And what do you think the title for the first question is? Sure enough: “Hu hattest þu?”. Literally "How call you" (In German the verb heißen can be both intransitive "Wie heißt du?", or transitive).
So indeed when the Saxons and the Angles landed in the south East of the British Isles they still used the Germanic construction, which other answers and comments have shown, is still in use today in many Germanic languages.
For instance, out of 91 riddles in the 10th century codex Exoniensis, I've counted 13 occurrences of “Saga hwæt ic hatte” (“Say how I[am] called”) and 5 “Frige hwæt ic hatte” (“Guess how I [am] called”).
When did it all change ?
This is trickier but it is supposed to be during Middle English, the period between 1066 and 1450. I'd like to cite two excerpts of the book Beginning Old English (Carole Hough and John Corbett ).
The harrying of the north: The Norman French, themselves descendants of Vikings, led by William the Conqueror, over-ran the Anglo-Saxon kingdoms and extended their territory throughout the Danelaw as far as the northern kingdom of Scotland.
And a similar one taking place after the harrying of the north:
This is consistent with a change occurring in Middle English. I find other explanations of the Celtic substrate re-emerging less convincing because of the 5 century gap between the invasion of the Anglo Saxons and the Exeter Book.
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