FromProto-Indo-European*prokʷe, from*pro- +*-kʷe (equivalent to Latinpro +-que). The expected form would accordingly be*proque. The second-p- is probably due to assimilation. Compare the reverse development inquinque,coquo, which was hindered here by the initial clusterpr-. A less likely explanation is borrowing fromOscan, where-𐌐𐌄(-pe) is the regular reflex.
The change from locative/ablative to accusative is caused by the adverbial suffix.
AIS:Sprach- und Sachatlas Italiens und der Südschweiz [Linguistic and Ethnographic Atlas of Italy and Southern Switzerland] –map 353: “accanto” – onnavigais-web.pd.istc.cnr.it
Carl Meißner; Henry William Auden (1894),Latin Phrase-Book[1], London:Macmillan and Co.
to be not far away:prope (propius, proxime) abesse
to be almost culpable:prope abesse a culpa
“prope”, inCharlton T. Lewis and Charles Short (1879),A Latin Dictionary, Oxford: Clarendon Press
“prope”, inCharlton T. Lewis (1891),An Elementary Latin Dictionary, New York: Harper & Brothers
"prope", in Charles du Fresne du Cange’sGlossarium Mediæ et Infimæ Latinitatis (augmented edition with additions by D. P. Carpenterius, Adelungius and others, edited by Léopold Favre, 1883–1887)
