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myriad (n.): 1550s, from Middle French myriade and directly from Late Latin myrias (genitive myriadis) "ten thousand," from Greek myrias (genitive myriados) "a number of ten thousand, countless numbers," from myrios (plural myrioi) "innumerable, countless, infinite; boundless," as a definite number, "ten thousand" ("the greatest number in Greek expressed by one word," Liddell & Scott say), of unknown origin; perhaps from PIE *meue- "abundant" (cognates: Hittite muri- "cluster of grapes," Latin muto "penis," Middle Irish moth "penis"). Specific use is usually in translations from Greek or Latin.
N: in nickname, newt, and British dialectal naunt, the -n- belongs to a preceding indefinite article an or possessive pronoun mine.

Other examples of this from Middle English manuscripts include a neilond ("an island," early 13c.), a narawe ("an arrow," c. 1400), a nox ("an ox," c. 1400), a noke ("an oak," early 15c.), a nappyle ("an apple," early 15c.), a negge ("an egg," 15c.). In 16c., an idiot sometimes became a nidiot, which, with still-common casual pronunciation, became nidget, which, alas, has not survived.

The process also worked in surnames, from oblique cases of Old English at "by, near," as in Nock/Nokes/Noaks from atten Oke "by the oak;" Nye from atten ye "near the lowland;" and see Nashville.

But it is more common for an English word to lose an -n- to a preceding a: apron, auger, adder, umpire, humble pie, etc. The mathematical use of n for "an indefinite number" is first recorded 1852, in to the nth power.
nth: 1852, in phrase to the nth, figurative use of a mathematical term indicating indefinite number, in which n is an abbreviation for number (n.).
umpty: 1905, "of an indefinite number," originally Morse code slang for "dash," influenced by association with numerals such as twenty, thirty, etc.