Etymology
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lad (n.)
c. 1300, ladde "foot soldier," also "young male servant" (attested as a surname from late 12c.), possibly from a Scandinavian language (compare Norwegian -ladd, in compounds for "young man"), but of obscure origin in any case. OED hazards a guess on Middle English ladde, plural of the past participle of lead (v.), thus "one who is led" (by a lord). Liberman derives it from Old Norse ladd "hose; woolen stocking." "The development must have been from 'stocking,' 'foolish youth' to 'youngster of inferior status' and (with an ameliorated meaning) to 'young fellow.'" He adds, "Words for socks, stockings, and shoes seem to have been current as terms of abuse for and nicknames of fools." Meaning "boy, youth, young man" is from mid-15c.
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Endymion 
beautiful youth, son of Jupiter and Calyce, beloved by Moon-goddess Selene, from Greek, perhaps literally "diver, plunger," from endyein "to enter into, sink into, plunge, dive," which was used in reference to the sun or stars setting into the sea. On this theory, he originally was a solar deity, a personification of the setting sun.
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donzel (n.)

titular designation of a young gentleman; "a young man of gentle or noble birth; young attendant, page, youth of good quality not yet knighted," 1580s, not found in Middle English, from Old French danzel, Old Italian donzello, from Medieval Latin domicellus, diminutive of Latin dominus "master," from domus "house" (from PIE root *dem- "house, household").

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rumble (n.)

"a deep, heavy, continuous rattling or dully roaring sound," as of thunder, late 14c., from rumble (v.). From 14c. to 17c. it also meant "confusion, disorder, tumult." The slang noun meaning "gang fight" is by 1946. The meaning "backmost part of a carriage" (typically reserved for servants or luggage) is from 1808 (earlier rumbler, 1801), probably from the effect of sitting over the wheels; hence rumble seat (1828), later transferred to automobiles.

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adolescence (n.)

"the age following childhood, the age of growing" (roughly the period from the 15th to the 21st year; or age 14 to 25 in males, 12 to 21 in females), early 15c., from Old French adolescence (13c.), from Latin adolescentia/adulescentia "youth, youthful people collectively," abstract noun from adulescentem "growing, youthful" (see adolescent (n.)). Adolescency (late 14c.) is slightly earlier.

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hobbledehoy (n.)

"clumsy or awkward youth," 1530s, of uncertain origin and the subject of much discussion. Suspicion has focused on French or Anglo-French, but no appropriate word has been found there. First element is probably hob in its sense of "clown, prankster" (see hobgoblin), the second element perhaps is French de haye "worthless, untamed, wild," literally "of the hedge."

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infancy (n.)
late 14c., "condition of babyhood," also "childhood, youth," from Anglo-French enfaunce and directly from Latin infantia "early childhood," from infantem "young child," literally "one unable to speak" (see infant). Restriction to the earliest months of life is a return to the etymological sense of the word but is a recent development in English. In old legal language it meant "condition of being a minor" and could mean any age up to 21.
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into (prep.)
Old English into "into, to, against, in," originally in to. It emerged in late Old English to do the work of the dative case inflections, then fading, that formerly distinguished, for instance, the notion of "in the house" from that of "into the house." Compare onto, unto. To be into (something) "be intensely involved in or devoted to" recorded by 1967 in American English youth slang.
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infantry (n.)
Origin and meaning of infantry
1570s, from French infantrie, infanterie (16c.), from older Italian or Spanish infanteria "foot soldiers, force composed of those too inexperienced or low in rank to be cavalry," a collective noun from infante "foot soldier," originally "a youth," from Latin infantem (see infant). Meaning "infants collectively" is recorded from 1610s. A Middle English (c. 1200) word for "foot-soldiers" was going-folc, literally "going-folk."
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knave (n.)

late Old English cnafa "boy, male child; male servant," from Proto-Germanic *knabon- (source also of Old High German knabo "boy, youth, servant," German knabe "boy, lad"); it is also probably related to Old English cnapa "boy, youth, servant," Old Norse knapi "servant boy," Dutch knaap "a youth, servant," Middle High German knappe "a young squire," German Knappe "squire, shield-bearer." Original sense unknown; Klein suggests the prehistoric meaning might have been "stick, piece of wood." For pronunciation, see kn-.

Sense of "rogue, rascal" is first recorded c. 1200, presumably via sense evolution from "a menial" to "one of low birth," and the low character supposed to be characteristic of such a condition. But through Middle English it kept also its non-pejorative meaning, as in knave-child (Scottish knave-bairn) "male child." In playing cards, "the lowest court card," 1560s.

Previously, the English equivalent of the French valet was normally known as Knave, in the sense of 'serving-lad'. In the seventeenth century it came to be called Jack, from the name properly applied to the Knave of trumps at All Fours. All Fours being a low-class game, the use of 'Jack' for 'Knave' was long considered vulgar. ('He calls the Knaves Jacks!', remarks Estella contemptuously in Dickens's Great Expectations.) When indices came in, it was obviously preferable to use 'J' rather than 'Kn' to avoid confusion with 'K' for King. Jack has since become the normal title of the lowest court, though 'Knave' can still be heard. [David Parlett, "A History of Card Games," 1991]
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