In a manner that is extremely unpleasant, painful, or terrible; terribly or damnably.
From 'hellish' (adjective) plus the adverbial suffix '-ly,' which converts adjectives into adverbs. 'Hellish' itself derives from 'hell' plus '-ish,' a pattern dating back to Old English.
English speakers naturally stack suffixes like '-ish' and '-ly' to create intensifying words—'hellishly' emerged as people needed to express extremes beyond basic 'bad' or 'awful,' especially in times of genuine hardship.
Complete word intelligence in one call. Free tier — 50 lookups/day.