*aidiją

Possibly from pre-Germanic *h₁oy-t-yó-m, a nominalized neuter of a gerundival/adjectival formation, *h₁oy-tó-s, built from the o-grade of Proto-Indo-European root *h₁ey- (Kroonen 2013: 8). Semantically, it would be: ‘passable’ > ‘passage, portage’ > ‘isthmus, strait’. Compare Ancient Greek ἰτός (itós) from the more typical zero-grade of the root, *h₁i-tó-s, Latin itus, and Sanskrit इत (ita) (Ringe 2017: 186; Watkins 2011: s.v. *ei-). The formally related *aiþaz may reflect a parallel *h₁óy-to-s formation with a semantic development ‘course, going’ > ‘oath’ (van Sluis et al. 2023: 219). Lehmann (1942: 127) considered this to be a dʰ-extension of the same root, *h₁oy-dʰ-yo-m, comparing Ancient Greek ῐ̓́θμᾰ (ĭ́thmă), which is related to ῐ̓σθμός (ĭsthmós) (Beekes 2010: 388), ultimately the source of English “isthmus” (via Latin, which borrowed it from Greek).