This wonderful word is the title of a new post at an Eudæmonist, and having looked it up I now know that it means (OED) ‘Characterized by creeping in or having crept in, esp. into a text.’
1673 Castell Let. in Nichols Lit. Anecd. 18th C. IV. 695 The first [text] he illustrates, Esa. ix. 1 where all condemn πιε [pie] as irreptitious. 1680 H. Dodwell Two Lett. (1691) 7 Where it [this design] is irreptitious and by way of surprize. 1868 Contemp. Rev. IX. 283 Omit ουδαμως [oudamos] which contradicts Micah, and is irreptitious from preceding αιδου [aidou].
The noun, by the way, is irreption, which my print OED marks with a dagger as obsolete (at that point having been last attested in 1649) but which is a living word in the online edition thanks to several 20th-century uses:
1926 G. W. S. Friedrichsen Gothic Version of Gospels 190 Previous to this there had been casual but continued irreptions from the Old Latin. [p.]249 The Gothic reading could.. be explained as a corruption due to the irreption of some parallel or reminiscent passage. 1974 Encounter Feb. 54/1 A protection against casual and deplorable irreptions creeping into the language.
And that last quote leads us right back to the Language