blunder

Definitions:

A gross mistake; a stupid error.
- Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874. (noun) (n.) (noun)
Confusion; disturbance.
- Newage Dictionary DB (noun) (n.) (noun)
A gross mistake.
- The american dictionary of the english language. By Daniel Lyons. Published 1899. (noun) (n.) (noun)
Blundering.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (adjective) (adj.) (adjective)
To make a gross mistake.
- The Clarendon dictionary. By William Hand Browne, Samuel Stehman Haldeman. Published 1894. (verb) (v. i.) (verb intransitive)
To mistake grossly; to err stupidly; to flounder about.
- Nuttall's Standard dictionary of the English language. By Nuttall, P.Austin. Published 1914. (verb, noun) (v. & n.) (verb, noun)
a stupid mistake
- Wordnet Dictionary DB (noun) (n.) (noun)
To make a bad mistake from stupidity, etc.; to act clumsily.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (verb) (v. i.) (verb intransitive)
To make a gross mistake, to flounder about.
- The american dictionary of the english language. By Daniel Lyons. Published 1899. (verb) (v. i.) (verb intransitive)
To mistake grossly; to err stupidly; to act without reflection.
- Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874. (verb) (v.) (verb)
Blunderingly.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (adverb) (adv.) (adverb)

Usage examples:

The decision was a blunder to begin with.
- Human Error by Raymond F. Jones
Let the truth be told: Davis was not the man to let such a blunder go unnoticed.
- Siege of Washington, D.C. by F. Colburn Adams
I forgot to mention one blunder we made.
- Astounding Stories of Super-Science, December 1930 by Various