When did 'wyrm' lose its meaning as dragon?
Solution 1:
According to the OED, wyrm (in the spelling worm) was first used to refer to the earthworm and similar creatures in Middle English, and even in early use it seemed to refer to a wide range of creepy crawlies. The Middle English Dictionary documents all of these meanings.
Dragon also emerged in Middle English as a loan word from French, transmitted through some combination of bestiaries, saints' lives like St. Margaret's, Biblical translations, and romances. I would consult the Middle English Dictionary quotations to see the sheer range of sources dragon appeared in.
Throughout Middle English the two terms coexisted, both referring to dragons. In the early modern period, the more modern meaning of worm has already taken over its earlier large and serpentine sense. The Lexicons of Early Modern English database shows the dominance of worm referring to insects and small serpents even in early entries. In Latin-English dictionaries they are often vermis, Latin for the modern worm. By Samuel Johnson's Dictionary (1755) he defines "worm" primarily in this way:
A small harmless serpent that lives in the earth.
A poisonous serpent.
Then parasites, then silkworms, then grubs, and then more figurative senses. Wyrm is absent. In contrast, dragon even in the 16th century was the primary translation of draconic words. The Dictionary of Sir Thomas Elyot (1538) glosses draco, -onis as "a dragon." Such usage continues down to Johnson's Dictionary, where "dragon" was defined in its modern meaning:
- A kind of winged serpent, perhaps imaginary, much celebrated in the romances of the middle age.
So save in specialized or archaic uses, dragon outpaces the draconic sense of worm by the 16th century.
As someone who has studied medieval literature for a long time, my sense is that worm was fast diminishing in Middle English save in texts that hearkened back to Old English usage like the alliterative Sir Gawain and the Green Knight (late 14th c.), whereas dragon became the preferred expression in popular saints' lives and romances. I consulted a few popular sources and one database to work through this hypothesis:
(1) Sir Thomas Malory's Le Morte D'Arthur (written ~1470, printed 1485), the Arthurian romance seminal to 19th and 20th century adaptations of King Arthur and his knights, features the dragon at several key points, including in a dream King Arthur has before fighting a giant. I'm writing from the Winchester MS and updating the spelling but not the syntax:
As the King was in his cog and lay in his cabin, he fell in a slumbering and dreamed how a dreadful dragon did drench much of his people, and come flying on wing out of the west parts. ...
Dragon occurs several times throughout the story. Worm and wyrm never appear. A later epic romance, Edmund Spenser's Faerie Queene (1590, 1596) fits the same pattern: dragon appears 13 times and worm only appears as "gealouse worme" and "worm-eaten." That's remarkable, since the Redcrosse Knight defeats a dragon at the end of book 1, so there was a perfect opportunity to fight a worm, but Spenser never chose that word for that purpose.
(2) The Wycliffe Bible are an early complete translation of the Bible completed by religious reformers under the direction of John Wycliffe. Dragon appears 47 times and usually refers to serpents or monsters:
(Revelation 12) : And another sign was seen in heaven; and lo! a great red dragon, that had seven heads [having seven heads], and ten horns, and in the heads of him seven diadems.
Worm appears 33 times, usually referring to small creatures:
(Isaiah 51:8) : For why a worm shall eat them so as a cloth, and a moth shall devour them so as wool; but mine health shall be without end, and my rightfulness into generations of generations.
So even around 1400 a series of translators were distinguishing worms and dragons.
(3) An Early English Books Online search for both worm/wyrm and dragon yields over 1500 results. I've looked at the first several, and so far the words appear to not overlap in meaning. Now, that doesn't preclude worm referring to large serpents in some specialized or literary uses, but even the OED entries for Shakespeare and Milton refer to small serpents. I can find no association with anything dragon-like in English after 1500, suggesting that the association after that point is rare or intentionally archaic:
a1616 Shakespeare Antony & Cleopatra (1623) v. ii. 238 Hast thou the pretty worme of Nylus there, That killes and paines not?
1667 Milton Paradise Lost ix. 1068 O Eve, in evil hour thou didst give eare To that false Worm .
Solution 2:
According to Etymologyonline:
dragon was borrowed in the middle of the 13 century, from Old French dragon and directly from Latin draconem (nominative draco) "huge serpent, dragon".
Here's a quotation from the Middle English text of 1275:
*St.Marg.(2) (Trin-C B.14.39)
"Ho sei a foul dragun ine þe hurne glide, berninde ase fur."
https://quod.lib.umich.edu/m/middle-english-dictionary/bibliography/BIB3314?rid=hyp.677.19990304t151115)
According to the above-mentioned Middle English Dictionary, "worm" was a polysemantic noun from 1275 to 1500.
The example of ''worm'' in the modern meaning is also quoted in 1250-1275:
c1275(?c1250) Owl & N.(Clg A.9) 601 : Ac wat etestu…Bute attercoppe & fule uliʒe, An wormes, ʒif þu miʒte finde Among þe uolde of harde rinde