blunder
n.a big or careless mistake. You use this when someone does something wrong because they were not thinking or were not careful.
n.a gross, stupid, or careless mistake resulting from ignorance or lack of deliberation.
“The goalkeeper made a terrible blunder in the final minute.”
“The company's decision to ignore the new market trends proved to be a costly blunder that led to bankruptcy.”
“Historians often debate whether the general's choice to divide his forces was a calculated risk or a strategic blunder that doomed the entire campaign.”
Often modified by adjectives like 'costly', 'tactical', or 'fatal'.
“The blunder follows a similar incident at last year's Great North Run in Newcastle, where medals mistakenly featured the Sunderland and Wear River – which is not part of the event.”
“He would later tell friends that introducing it had been his "greatest blunder." Then came another major surprise.”
Inherited from Middle English blundren, blondren (verb) and blunder, blonder (“disturbance, strife”), from the verb; 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”), Icelandic blunda (“to nap; doze”). Related to English blind.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.