proprius

Most likely from Proto-Italic *pro- + *prijos (“own, dear”), from Proto-Indo-European *priHós (“dear”). Since pro- can hardly be explained as a nominal prefix, Forssman (2004) explains it as having been extended to the adjective from the verb propriāre, once-attested as propriāssit. Cognate to Sanskrit प्रिय (priyá, “dear”), Russian прия́тель (prijátelʹ, “buddy, mate”) and Russian прия́тный (prijátnyj, “pleasant”), Welsh rhydd (“free”), English free.

Other suggestions include:

for *proprīvus, from the root of prīvus (“individual”), from Proto-Indo-European *prey-wo-s (“being in front”).
for *propatrius, from pro- +‎ pater, meaning “from one's forefathers”, taking Ancient Greek προπάτωρ (propátōr, “forefather”) as comparison.

(Classical) IPA^(key): /ˈpro.pri.us/, [ˈprɔpriʊs̠] or IPA^(key): /ˈprop.ri.us/, [ˈprɔpriʊs̠]
(modern Italianate Ecclesiastical) IPA^(key): /ˈpro.pri.us/, [ˈprɔːprius] or IPA^(key): /ˈprop.ri.us/, [ˈprɔprius]

proprius (feminine propria, neuter proprium); first/second-declension adjective

appropriate to oneself: (one's) proper, (one's) own (inalienably)
belonging to oneself as property, or goods: (one's) own (alienably) [ quotations ▼]
particular to one individual, group, or case: characteristic, distinctive, eccentric, idiosyncratic, individual, specialized
not shared with others: personal, private, exclusive [ antonym ▲]
Antonym: commūnis
(of activities) proper to, characteristic of a given set of persons or things
(of words) proper, literal, strict [ antonyms ▲]
Antonyms: trā(ns)lāticius, trā(ns)lātus

Case / Gender	Masculine	Feminine	Neuter	Masculine	Feminine	Neuter
Number	Singular			Plural
Nominative	proprius	propria	proprium	propriī	propriae	propria
Genitive	propriī	propriae	propriī	propriōrum	propriārum	propriōrum
Dative	propriō	propriae	propriō	propriīs
Accusative	proprium	propriam	proprium	propriōs	propriās	propria
Ablative	propriō	propriā	propriō	propriīs
Vocative	proprie	propria	proprium	propriī	propriae	propria