Dictionary.com
Thesaurus.com

dunno

American  
[duh-noh] / dəˈnoʊ /
Informal.

  1. don't know.

    Who did it? I dunno!
dunno British  
/ də-, dʊ-, dʌˈnəʊ /

contraction

  1. (I) do not know

"Collins English Dictionary — Complete & Unabridged" 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Etymology

Origin of dunno

First recorded in 1835–45; a phonetic spelling representing the pronunciation of don't know in continuous rapid speech

Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.