1880, from Latinized form of Greek ephebikos "of or for an ephebe," from ephebos "one arrived at puberty, one of age 18-20," from epi "upon" (see epi-) + hēbē "early manhood," from PIE *yegw-a- "power, youth, strength." In classical Athens, a youth of 18 underwent his dokimasia, had his hair cut off, and was enrolled as a citizen. His chief occupation for the next two years was garrison duty.
youth of Abydos, lover of Hero. He swam nightly across the Hellespont to visit her in Sestos, on the Thracian side, until he drowned. The name is from Greek Leiandros, literally "lion-man," from leon "lion" + anēr (genitive andros) "man" (from PIE root *ner- (2) "man").
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]
subdivision of an army, 1630s, from French brigade "body of soldiers" (14c.), from Italian brigata "troop, crowd, gang," from brigare "to brawl, fight," from briga "strife, quarrel," perhaps of Celtic (compare Gaelic brigh, Welsh bri "power"), from PIE root *gwere- (1) "heavy." Or perhaps from Germanic.
"beautiful young man," 1620s, probably via French Adonis (15c.), from Greek Adōnis, name of the youth beloved by Aphrodite, from Phoenician adon "lord," probably originally "ruler," from base a-d-n "to judge, rule." Adonai is the Hebrew cognate.