Etymology
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urchin (n.)
c. 1300, yrichon "hedgehog," from Old North French *irechon (cognate with Picard irechon, Walloon ireson, Hainaut hirchon), from Old French herichun "hedgehog" (Modern French hérisson), formed with diminutive suffix -on + Vulgar Latin *hericionem, from Latin ericius "hedgehog," enlarged form of er, originally *her, from PIE root *ghers- "to bristle" (source also of Greek kheros "hedgehog;" see horror).

Still used for "hedgehog" in non-standard speech in Cumbria, Yorkshire, Shropshire. Applied throughout 16c. to people whose appearance or behavior suggested hedgehogs, from hunchbacks (1520s) to goblins (1580s) to bad girls (1530s); meaning "poorly or raggedly clothed youngster" emerged 1550s, but was not in frequent use until after c. 1780. Sea urchin is recorded from 1590s (a 19c. Newfoundland name for them was whore's eggs); Johnson describes it as "a kind of crabfish that has prickles instead of feet."
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sea-urchin (n.)

1590s; see sea + urchin. A 19c. Newfoundland name for them was whore's eggs); Johnson describes it as "a kind of crabfish that has prickles instead of feet." 

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sand-dollar (n.)

type of flat sea-urchin, 1884, so called for its shape; see sand (n.) + dollar (n.). Cake-urchin (1853) and sand cake were other old names for it.

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guttersnipe (n.)
also gutter-snipe, 1857, from gutter (n.) + snipe (n.); originally Wall Street slang for "streetcorner broker," attested later (1869) as "street urchin," also "one who gathers rags and paper from gutters." As a name for the common snipe, it dates from 1874 but is perhaps earlier.
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echinoderm (n.)
1834, from Modern Latin Echinodermata, name of the phylum that includes starfish and sea urchins, from Latinized form of Greek ekhinos "sea urchin," originally "porcupine, hedgehog" (see echidna) + derma (genitive dermatos) "skin," from PIE root *der- "to split, flay, peel," with derivatives referring to skin and leather. So called from its spiky shell. Related: Echinodermal.
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gamin (n.)

"street urchin," 1837, from French gamin (late 18c.), perhaps from Berrichon dialect gamer "to steal." Introduced in English in translations of Hugo.

Un groupe d'enfants, de ces petits sauvages vanu-pieds qui ont de tout temps battu le pavé de Paris sous le nom éternel de gamins, et qui, lorsque nous étions enfants aussi, nous ont jeté des pierres à tous, le soir, au sortir de classe, parce que nos pantalons n'étaient pas déchirés; etc. [Hugo, "Notre-Dame de Paris"]
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boy (n.)

mid-13c., boie "servant, commoner, knave" (generally young and male); c. 1300, "rascal, ruffian, knave; urchin," mid-14c. as "male child before puberty" (possibly an extended sense from the "urchin" one). A word of unknown origin.

Possibly from Old French embuie "one fettered," from Vulgar Latin *imboiare, from Latin boia "leg iron, yoke, leather collar," from Greek boeiai dorai "ox hides." (Words for "boy" double as "servant, attendant" across the Indo-European map -- compare Italian ragazzo, French garçon, Greek pais, Middle English knave, Old Church Slavonic otroku -- and often it is difficult to say which meaning came first.)

But it also appears to be identical with East Frisian boi "young gentleman," and perhaps with Dutch boef "knave," from Middle Dutch boeve, perhaps from Middle Low German buobe. This suggests a gradational relationship to babe. For a different conjecture: Used slightingly of young men in Middle English, also in familiar or contemptuous use of criminal toughs or men in the armed services. In some local uses "a man," without reference to age (OED lists "in Cornwall, in Ireland, in the far West of the U.S."). Meaning "male negro slave or Asian personal servant of any age" attested from c. 1600. Extended form boyo is attested from 1870. Emphatic exclamation oh, boy is attested by 1917. Boy-meets-girl "typical of a conventional romance" is from 1945; the phrase itself is from 1934 as a dramatic formula. Boy-crazy "eager to associate with males" is from 1923.

In Old English, only the proper name Boia has been recorded. ME boi meant 'churl, servant' and (rarely) 'devil.' In texts, the meaning 'male child' does not antedate 1400. ModE boy looks like a semantic blend of an onomatopoeic word for an evil spirit (*boi) and a baby word for 'brother' (*bo). [Liberman]
A noticable number of the modern words for 'boy', 'girl', and 'child' were originally colloquial nicknames, derogatory or whimsical, in part endearing, and finally commonplace. These, as is natural, are of the most diverse, and in part obscure, origin. [Buck]
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*pere- (1)

*perə-, Proto-Indo-European root meaning "to produce, procure" and yielding and derived words in diverse senses; possibly related to *pere- (2) "to grant, allot."

It forms all or part of: ante-partum; apparatus; apparel; biparous; disparate; emperor; empire; heifer; imperative; imperator; imperial; juniper; multiparous; nulliparous; oviparous; para- (2) "defense, protection against; that which protects from;" Parabellum; parachute; parade; parados; parapet; parasol; pare; parent; -parous; parry; parturient; poor; post-partum; preparation; prepare; primipara; puerperal; rampart; repair (v.1) "to mend, put back in order;" repertory; separate; sever; several; spar (v.); viper; vituperation; viviparous.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit prthukah "child, calf, young of an animal;" Greek poris "calf, bull;" Latin parare "make ready, prepare," parire "produce, bring forth, give birth to;" Czech spratek "brat, urchin, premature calf;" Lithuanian periu, perėti "to brood;" Old High German farro, German Farre "bullock," Old English fearr "bull."

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echidna (n.)

Australian egg-laying hedgehog-like mammal, 1810, said to have been named by Cuvier, usually explained as from Greek ekhidna "snake, viper" (also used metaphorically of a treacherous wife or friend), from ekhis "snake," from PIE *angwhi- "snake, eel" (source also of Norwegian igle, Old High German egala, German Egel "leech," Latin anguis "serpent, snake").

But this sense is difficult to reconcile with this animal (unless it is a reference to the ant-eating tongue). The name perhaps belongs to Latin echinus, Greek ekhinos "sea-urchin," originally "hedgehog" (in Greek also "sharp points"), which Watkins explains as "snake-eater," from ekhis "snake." The 1810 Encyclopaedia Britannica gives as the animal's alternative name "porcupine ant-eater." Or, more likely, the name refers to Echidna as the name of a serpent-nymph in Greek mythology, "a beautiful woman in the upper part of her body; but instead of legs and feet, she had from the waist downward, the form of a serpent," in which case the animal was so named for its mixed characteristics (early naturalists doubted whether it was mammal or amphibian).

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