mid-14c. as a present participle adjective; mid-15c. as a verbal noun, from blunder (v.). Related: Blunderingly.
mid-14c., "to stumble about blindly," from a Scandinavian source akin to Old Norse blundra "shut one's eyes," from PIE root *bhlendh- (see blind). Meaning "make a stupid mistake" is first recorded 1711. Related: Blundered; blundering.
mid-14c., apparently from blunder (v.), though of about the same age.