"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).
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit antara- "interior;" Greek en "in," eis "into," endon "within;" Latin in "in, into," intro "inward," intra "inside, within;" Old Irish in, Welsh yn, Old Church Slavonic on-, Old English in "in, into," inne "within, inside."
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]