Young Avestan pašne and its etymology
Young Avestan pašne is attested ten times in constructions with genitive and accusative used to indicate the position of the figures at the moment of the worship of various deities. The only occurrence of the construction with the accusative in Yt 5.108 must be due to an analogy with the upa + accusative construction that occurs earlier in the text. The author argues that pašne does not mean 'in the sight of', but rather 'at the shore of' and is a loc. sg. of pašna- n. 'bank, shore, side'. This word is most likely identical with pašna- 'eyelid' or 'eyelash'. Similar semantic development can be found in Skt. vártman- 'track of a wheel, way' (RV+) besides 'eyelid' (AV) or Skt. vartaní- 'way, wheel rim, track of a wheel' (RV+) and 'eyelashes' (ŚBr.), with the meanings 'eyelid' and 'eyelashes' being derived from the meaning 'circumference'.
From the etymological point of view, Young Avestan pašna- can be compared with other Indo-Iranian words for 'side, flank', like Skt. pā́jas- n. 'side, surface', Khot. pāysa- 'surface, breast', Sogd. p'z 'face', Oss. faz / fazæ 'half, side; back, buttocks'. The semantic development 'side' > 'shore' is trivial, cf. English river side. These words are derived from the PIE root *peh₂ǵ- 'to attach, fix' (Gk. πήγνυμι 'to fix, join, congeal', Lat. pangō 'to attach, join', etc.), PIIr. *paHj́- (this root loses its laryngeal, when followed by a consonant, cf. Skt. pajrá- 'solid, strong, firm' (RV) < PIE *peh₂ǵ- ro-). Avestan pašna- goes back to PIIr. *paj́-na- < *paHj́-na-, which, in theory, can reflect three PIE formations: *peh₂ǵ-no- (cf. Lat. pīgnus 'pledge, hostage'), *peh₂ǵ-mn-o-, thematicized variant of *peh₂ǵ- m(e)n- (cf. Gk. πῆγμα 'joint together, stage, scaffold') and *peh₂ǵ-(e)n- (cf. Lat. pāgina 'side, sheet of paper'). Of these options, the latter is clearly preferable.