Etymology tree
Proto-Indo-European *h₁en-
Proto-Indo-European *h₁en-
Proto-Indo-European *h₁én
Proto-Italic *en
Old Latin en
Latin in
Latin in-
Proto-Italic *dūros
Latin dūrūs
Proto-Indo-European *-h₂
Proto-Indo-European *-éh₂
Proto-Indo-European *-yéti
Proto-Indo-European *-eh₂yéti
Proto-Italic *-āō
Latin -ō
Latin dūrō
Latin indūrō
From in- + dūrō (“harden, endure”).