Winston Churchill used this term to refer to his depression
The black-dog has bitten me today
by tigertrakstar November 19, 2007
n. (possibly of Irish origin): A bad mood, characterized by anger, depression, or a mixture of the two. Emotional malaise. An antisocial mood. Sometimes synonymous with a 'purple rain.'
"I'm just telling you this because I don't like seeing you in a black dog every day." -Leo Shannon (Da Vinci's Inquest.)

"Best to stay away from me today, because I feel a black dog coming on."
by parisofpriam February 10, 2006
A kick-ass song by Led Zeppelin that famously starts off with the lyrics "Hey, hey, mama, said the way you move // Gonna make you sweat, gonna make you groove!" sung in acappella by Robert Plant, just before Pagey and Bonzo kick it into high gear with the lead guitar and drums, respectively.

The song supposedly got its name from a stray black dog that kept walking in and out of the studio while the song was being recorded.
Dude #1: Is that "Black Dog?"
Dude #2: Yeah, man! Led Zepp rules!
Dude #1: Sweet! Crank it up!
by T-Ro May 13, 2006
Song by Led Zeppelin that rocks. However, it never once mentions "black dog". Probably named so because of the afore-mentioned definition. So I guess it makes you feel like that. Or because Robert Plant was on crack. That crazy bastard.
Led Zeppelin was cool. Black dog was cool too. I love your Mom.
by Jim E. Junk March 11, 2006
To kick a pirate - like character, a genuine scoundrel, out of a tavern after he orders a double rum. When caught they must be physically harmed in some manner. Time of origin: 18th century Britain. Popularized in the classic Disney movie Treasure Island.
Long John Silver: "Next time that scoundrel comes by my tavern I'll black dog em'."
by Jen Gun December 23, 2008
An apparition seen after copious amounts of heavy drug use.
I was so trashed off that E, I thought I saw the black dog.
by Dutch Hawkins May 11, 2009