Just as it says above. And no, it is not short for デコレーション, or decoration.
It's used in this context; "おぬしデコじゃのーう。デコデコじゃのーう。"
(Yes, I know that sounds a bit creepy, but that's what it is.) And that is some strange dialect for "You are デコ. You are デコデコ."
I searched, my best guess for what デコ...