1550s, "the number of 10,000," also "an indefinitely great number," from 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" (source also of Hittite muri- "cluster of grapes," Latin muto "penis," Middle Irish moth "penis"). Beekes offers "no etymology." The numerically specific use is usually in translations from Greek or Latin.

"numberless, multitudinous," c. 1800, from myriad (n.).

popularized 1871, American English, (identified throughout the 1870s as "a California word") "young street rowdy, loafer," especially one involved in violence against Chinese immigrants, "young criminal, gangster;" it appears to have been in use locally from a slightly earlier date and may have begun as a specific name of a gang:

The police have recently been investigating the proceedings of a gang of thieving boys who denominate themselves and are known to the world as the Hoodlum Gang. [San Francisco Golden Era newspaper, Feb. 16, 1868, p.4]

Of unknown origin, though newspapers of the day printed myriad fanciful stories concocted to account for it. A guess perhaps better than average is that it is from German dialectal (Bavarian) Huddellump "ragamuffin" [Barnhart].

What the derivation of the word "hoodlum" is we could never satisfactorily ascertain, though several derivations have been proposed; and it would appear that the word has not been very many years in use. But, however obscure the word may be, there is nothing mysterious about the thing; .... [Walter M. Fisher, "The Californians," London, 1876]