Probably from Old Norse 'blundr' (drowsy, sluggish) — the same dullness metaphor that produced 'blunder.'
Having a flat or rounded edge or point; not sharp; direct and straightforward in speech, often to the point of rudeness.
Probably from Old Norse 'blundr' (dozing, drowsy, half-asleep) or the related verb 'blunda' (to shut the eyes, to doze), from Proto-Germanic *blundaz (dull). The connecting thread is dullness: a sleeping person is mentally blunted, a dull blade is physically blunted, and a blunt speaker is socially unsharpened — all share the Proto-Germanic sense of diminished keenness. An alternative links it to a Scandinavian root meaning 'muddy' or 'opaque
'Blunt' and 'blunder' likely share the same Norse root — both involve dullness or clumsiness. A blunder is a 'dull' mistake, made from lack of sharpness. The sense evolution from 'dull-witted' to 'not physically sharp' to 'rudely direct' shows how the same core metaphor — lack of refinement or edge — branched in multiple directions