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black dog

American  

noun

Informal.
  1. melancholy; despondency; the blues.

    The black dog is over him.


black dog British  

noun

  1. informal depression or melancholy

"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

Etymology

Origin of black dog

First recorded in 1700–10