noun Slang.
a dog, especially a mongrel.
a stupid or foolish person; simpleton.
Also, mut.

1900–05, Americanism; short for muttonhead Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
mutt (mʌt)
1.  an inept, ignorant, or stupid person
2.  a mongrel dog; cur
[C20: shortened from muttonhead]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

1901, "stupid or foolish person," probably a shortening of muttonhead (1803); meaning "a dog," especially "a mongrel" is from 1904, originally simply a term of contempt.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
The creatures described here are all uncompromisingly real, whether they be
  purebred or pound mutt.
It is said that if you want a healthy dog, make sure to get a mutt.
CM is a mutt, and we're not exactly sure what she has in her.
Besides cloning a great mutt, in other words, the project may contribute
  insight into the old question of nature vs nurture.
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