Learn English – What does it mean to be “worth someone’s keep”

Black dog is an oft-used phrase to mean depression.

Here's a link that attributes the phrase to Winston Churchill, but I suspect he didn't invent it. I like the way they described it though:

"Black Dog" was Churchill's name for his depression, and as is true with all metaphors, it speaks volumes. The nickname implies both familiarity and an attempt at mastery, because while that dog may sink his fangs into one's person every now and then, he's still, after all, only a dog, and he can be cajoled sometimes and locked up other times.

This paper (PDF, sorry), claims several references in the 1800's. Interestingly, the way they describe it (being ghostly and following people around) makes one wonder if it isn't ultimately related to the English folklore character Black Shuck.

The idiom seems to be the opposite of keep one's shirt on. After a bit of Googling, I found the following reason which makes perfect sense to me:

So what does all this shirt business have to do with being annoyed? A Dictionary of Slang and Unconventional English by Eric Partridge suggests that it comes from the custom of taking off one’s shirt before fighting. I wouldn't argue with that.