The aim of the present paper, offered to Prof. Nikolai Kazansky as a token of respect and homage for his contribution to Indo-European linguistics, is to explore the etymology of an Old Lithuanian prepositional locution uß oßcʒių 'on the back side' used twice by Jonas Bretkūnas in two different passages of his translation of the Bible (1590). It can be argued that the genitive plural oßcʒių reflects a feminine noun *oščios 'backside' limited to the plural (plurale tantum) and is based on a Baltic feminine noun *āź-tjā-. Historically, *āź-tjā- seems to reflect the combination of a lengthened allomorph of the preposition až(u)- 'behind' and a suffix *-ti̯o- which enjoyed a certain productivity in the prehistory of the Baltic languages. The long vowel of the preposition *āź- is striking, however, and remains completely unparalleled in Lithuanian, where až(u)- is only attested with a short vowel, or with lengthening of the second vowel ažúo-, but not with a first long vowel *āž-, which in turn appears in some Latvian dialects (āz). Taken at face value, the alternation between *ăź- and *āź- preserved in the prepositional locution uß oßcʒių recalls that between *pă- (Lithuanian pa-) and *pā- (Lithuanian pó-), but the difficulty is how to clarify the function of its initial lengthening in contrast with the more common alternation *aź(u)- / aźúo-. On the other hand, the Baltic suffix -tjā (< Proto-Indo-European *-ti̯eh₂-) is well attested in Baltic, in particular in the formation of abstract feminine nouns derived from prepositions (cf. for example Lithuanian apačià 'lower parts, undersides', Latvian apakša 'lower part' < Baltic *apa-tjā < Proto-Indo-European *(H)opo-ti̯eh₂-).