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1. relating to or characteristic of hell or the underworld:

｟書｠ 地獄の

e.g. the infernal regions

e.g. the infernal heat of the forge.

2. attributive informal irritating and tiresome (used for emphasis):

｟やや古｠ ひどい, いまいましい

e.g. you're an infernal nuisance.

infernally |inˈfərnlē| adverb

late Middle English: from Old French, from Christian Latin infernalis, from Latin infernus ‘below, underground’, used by Christians to mean ‘hell’, on the pattern of inferni (masculine plural) ‘the shades’ and inferna (neuter plural) ‘the lower regions’.