deet or DEET (d?t)n. A colorless, oily liquid, C12H17NO, that is used as an insect repellent.[From d.t., abbr. of diethyl toluamide.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.DEET (di?t) n acronym for (Chemistry) diethyl(meta)toluamide; an insect repellentCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014