Not planned or thought out in advance. See Synonyms at extemporaneous.(adjective)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "unpremeditated" in a sentence
"That colloquial use of the word “intuition” refers to an unpremeditated insight that just happens to turn out to be true."
""Like most inexperienced people," ran his notes, "I was astonished at the reported feats of men in war; I believed they were exaggerated, and that there was a kind of unpremeditated conspiracy of silence about their real behaviour."
"They think the attack last week was unpremeditated."