Etymology
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reptile (n.)

late 14c., "creeping or crawling animal; one that goes on its belly on the ground on small, short legs," from Old French reptile (early 14c.) and directly from Late Latin reptile, noun use of neuter of reptilis (adj.) "creping, crawling," from rept-, past-participle stem of repere "to crawl, creep." This is reconstructed to be from PIE root *rep- "to creep, crawl" (source also of Lithuanian rėplioti "to creep").

Used of persons of abject, groveling, or mean character from 1749. As an adjective, c. 1600, "creeping or crawling," hence, of persons, "low, mean" (1650s). Also sometimes used 18c. of creeping plants.

The precise scientific sense of the noun began to develop mid-18c., but the word also was used at first of animals now known as amphibians, including toads, frogs, salamanders. The separation of Reptilia (1835 as a distinct class) and Amphibia took place early 19c.; popular use lagged, and reptile still was used late 18c. with sense "An animal that creeps upon many feet" [Johnson, who calls the scorpion a reptile], sometimes excluding serpents. The Old English word for "reptile" was slincend, related to slink.

And the terrestrial animals may be divided into quadrupeds or beasts, reptiles, which have many feet, and serpents, which have no feet at all. [Locke, "Elements of Natural Philosophy," 1689]
An inadvertent step may crush the snail
That crawls at ev'ning in the public path ;
But he that has humanity, forewarn'd,
Will tread aside, and let the reptile live.
[Cowper, "The Task," 1785]
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reptilian (adj.)

"of, resembling, or characteristic of reptiles," 1835, from reptile + -ian. Transferred meaning "malignant, cold, underhanded" is by 1859.

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Reptilia (n.)

in biology, the class of cold-blooded, scaled vertebrates including the reptiles proper, mid-17c., from Latin plural of reptile (see reptile; also see -a (2)).

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irreption (n.)
"a creeping in," 1590s, from Late Latin irreptionem (nominative irreptio), noun of action from past participle stem of Latin irrepere, from assimilated form of in- "in" (from PIE root *en "in") + repere "to creep" (see reptile).
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obreption (n.)

"the obtaining or trying to obtain something by craft or deception," 1610s, from Latin obreptionem (nominative obreptio)  "a creeping or stealing on," noun of action from past-participle stem of obrepere "to creep on, creep up to," from ob "on, to" (see ob-) + repere "to creep" (see reptile). Opposed to subreption, which is to obtain something by suppression of the truth. Related: Obreptious.

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mosasaurus (n.)

carnivorous marine reptile of the Cretaceous period, 1830, from Latin Mosa "the river Meuse" (Dutch Maas) + -saurus. the first fossils of the ancient reptile were discovered 1760s in a chalk quarry near Maastricht, on the Meuse.

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ichthyosaur (n.)
extinct aquatic reptile, 1830, Modern Latin, from Latinized form of Greek ikhthys "fish" (see ichthyo-) + -saurus. Related: Ichthyosaurus (1819); ichthyosaurian.
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herpetology (n.)
"study of reptiles," 1816, from French herpétologie (18c.), coined from Greek herpeton "reptile," literally "creeping thing," from herpein "to creep" (see serpent) + French -logie (see -logy). Related: Herpetologist; herpetological.
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plesiosaurus (n.)

extinct gigantic long-necked marine reptile, 1825, from Modern Latin Pleisiosaurus (1821), coined by English paleontologist William Daniel Conybeare (1787-1857) from Greek plēsios "near" (related to pelas "near, nearby," and probably from PIE *pelh- "to approach") + sauros "lizard" (see -saurus). It was one of the earliest "antediluvian" reptile fossils discovered in the scientific age and was so called for being more like a modern lizard than the ichthyosaur fossils that had been found a few years earlier in the same rock in England.

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saurian (n.)
reptile of the order Sauria, 1819, from Modern Latin sauria "the order of reptiles," from Greek sauros "lizard" (see -saurus). Sauropod is 1891, from Modern Latin sauropoda (O.C. Marsh, 1884), second element from Greek pous "foot" (from PIE root *ped- "foot").
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