In a manner that causes or involves loss, damage, or harm; injuriously.
From the adjective damnous plus the adverbial suffix -ly. A rare formation from an already obscure adjective, making it doubly uncommon in modern English.
This adverb is so rare that it barely exists in the English language — it's like trying to use 'damnously' when everyone around you is saying 'harmfully,' and it makes perfect sense why the word never took off.
Complete word intelligence in one call. Free tier — 50 lookups/day.