Adverb contracted from the ablative īnferā (parte), of īnferus. Compare Faliscan 𐌉𐌅𐌓𐌀 (ifra), Oscan 𐌇𐌖𐌍𐌕𐌓𐌖𐌔 (huntrus), Umbrian 𐌇𐌏𐌍𐌃𐌓𐌀 (hondra). The accusative is from the adverbial derivation.