"According to Professor Jonathan Lighter’s Historical Dictionary of American Slang, it probably derives from Major Hoople, who was a character in a once-famous cartoon strip entitled Our Boarding House, which featured the goings-on at Martha Hoople’s rooming establishment. It was written and drawn by Gene Ahern and began to appear in September 1921..."
"...It would not have been possible for Al Swearengen (Deadwood) to have used the word in 1876, 40+ years before Gene Ahern invented the character (Major Hoople) and a hundred years before it was first recorded in print. The producer and head of the scriptwriting team (HBO: Deadwood), David Milch, has been reported as saying in essence that he picked something out of the air to serve as a suitable insult without great concern for its etymology. It seems he must have heard it somewhere and it came conveniently back to mind while writing the scripts. It’s definitely an anachronism."
In the show, these people often tend to be drunk or drinking, but not always.
Go get those hoople heads out of the street, before they get run over.