mid-14c., "to stumble about blindly," from a Scandinavian source akin to Old Norse blundra "shut one's eyes," perhaps from Proto-Germanic *blinda- "blind" (see blind (adj.)). Meaning "make a stupid mistake" is first recorded 1711. Related: Blundered; blundering.
late 14c., blonder, blunder, "disturbance, strife; trouble, distress;" apparently from blunder (v.). Original sense obsolete. Meaning "a mistake made through hurry or confusion" is from 1706.