early 15c., "gradually;" mid-15c., "unskillfully;" 1550s, "stupidly," from blunt (adj.) + -ly (2). Meaning "directly, abruptly" is from 1570s.
he spoke bluntly
blunder
blunderbuss
blunderful
blundering
blunt
bluntly
bluntness
blur
blurb
blurry
blurt