Bluntly comes from blunt + -ly; blunt is from Middle English blunt ('dull, obtuse'), of obscure origin, perhaps Scandinavian. The historical trail runs through blunt in Middle English. Morphologically, bluntly is transparent: it draws on blunt + -ly. Used as an adverb, bluntly now means "in a direct, frank manner without softening the message", so the present definition can be read against the older forms and meanings. The same lexical family is also visible in related forms such as blunt and bluntness. That uncertainty is part of the entry: the safest account is the attested form and the limited comparisons preserved with it.