In a December 11, 1991 interview, Barks told Geoffrey Blum about the origin of the word: "The only person I ever heard use the word was my first father-in-law. He was an Englishman, and when he was talking to kids, he would tell them scary stories about these big bohimatons that'd come down the stairs in the dark. And I think I did find it in a dictionary one time." (Barks gets the dictionary, but cannot find the word.)
In 1978, Barks told in an interview with Michael Barrier that he had picked the word up from an Englishman he had known in a logging camp, when he was young: "That was a word he would use once in a while, to describe something of huge size."
When Geoffrey Blum asked Barks whether it was his first-father-in-law or a man in a logging camp, Barks answered: "Well, he was a logger."
Searching for the word, Per Starbäck wrote on the Disney comics Mailing List: "It's not to be found in the Oxford English Dictionary at least, and not in a bunch of other dictionaries I've searched, including several dialectology books. Maybe it was just a word peculiar to this logger or father-in-law or whatever he was. It's not that uncommon with words that are only used in a family or a small circle of friends. Maybe the origin is "behemoth" ("something of monstrous size or power") from Job 40:15-24? That's the closest I can find with a related meaning."