When I was young, my father didn't want to swear in front of the kids. He used to say arse, instead of ass. After a few years, the kids knew what he meant when he said arse, so he had the ingenuity of changing arse to harse. That's the best explanation I can come up with, but most likely it is just because he is a harse and likes to make up words.
Glenn, I know you are a bastard, but you don't have to be a harse too.