In a manner that shows something is regrettable or worthy of disapproval; while deploring or lamenting.
From the adjective form of deplored (past participle used adjectivally) + -ly. A rare construction showing how English can add -ly to almost any adjective form.
This word is so rare it barely appears in modern English, but it's technically valid—it shows how English's suffix system is infinitely productive, allowing native speakers to create new adverbs on the fly even if dictionaries never catch them.
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