a person who has no opinion or is undecided, as in answering an item on a public-opinion poll.
Origin of don't-know
First recorded in 1885–90
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
a person who has not reached a definite opinion on a subject, esp as a response to a questionnaire
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