The phrase "butt end" refers to the rear or opposite side of something, usually an object or tool. It is the part that you hold or grip when using it. Full definition
However, I'm guessing any big signing will be at butt end of window. (justarsenal.com)
The handles are made of with rubber grips and a metal butt end. (geekwrapped.com)
The rubber keeps the grip comfortable so you can get a nice grip on your tool while the metal butt end means it can take whacks from a standard hammer. (geekwrapped.com)