Busy as a one-armed paper hanger meaning

If you've ever hung wallpaper, you know it can get pretty intense when you've spread the wallpaper paste and you then have to hoist it up the ladder to the corner of the ceiling to hang it on the wall. It is tricky to get it perfectly lined up with the vertical plumb line, and get it glued evenly to the wall, without bubbles or lumps. And it all has to be done in fairly short time, before the paste dries. It's a tricky job at the best of times, when you have two arms. Imagine the hassle of doing all that at once -- with one arm!