Blunder
Middle English: probably of Scandinavian origin and related to blind.
wiktionary
From Middle English blunder, blonder(“disturbance, strife”), from Middle English blundren, blondren(verb), which itself is partly from Middle English blondren, a frequentative form of Middle English blonden, blanden("to mix; mix up"; corresponding to blend + -er); and partly from Middle English blundren, a frequentative form of Middle English blunden(“to stagger; stumble”), from Old Norse blunda(“to shut the eyes; doze”).
Cognates include Norwegian blunda(“to shut the eyes; doze”), dialectal Swedish blundra(“to act blindly or rashly”), Danish blunde(“to blink”) or blunde(“to take a nap”). Related to English blind.
etymonline
blunder (v.)
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.
blunder (n.)
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.
