dumb-ass or dumb·ass [duhm-as] WORD ORIGIN noun Slang: Vulgar. a thoroughly stupid person; blockhead. Liberaldictionary.com
Origin of dumb-ass 1970–75; Americanism Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019 British Dictionary definitions for dumbass dumb-ass noun a stupid person adjective extremely stupid 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 Word Origin and History for dumbass n.
by 1959, from dumb “stupid” + ass (n.2).
Online Etymology Dictionary, © 2010 Douglas Harper