blunder (v.)
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.
blunder (n.)
mid-14c., apparently from blunder (v.), though of about the same age.