I agree this term came from "Deadwood", but I don't think it means drunk or drunkard specifically. I think a more accurate definition would be clueless bastard or loser or lowlife, or particularly a combination of these.
In the show, these people often tend to be drunk or drinking, but not always.
1 It derives from Hoople North Dakota.
Simple isn't it.
2 One other common 1800's use was also because of the hoop on a miners head, they used lights on hoops to go in shafts.
A drunkard, likely an old variant of "hop-head". Re comments about Deadwood: remember the character of Al Swearingen is described as a Limey, so this is quite possibly a usage he brought with him from England.