The end of the measuring tape that starts at zero. All an individual has to do is hold the "dummy end", while the other person stretches out the measuring tape and does the actual measuring. Commonly used to describe a complete moron that is incompetent at a given task.
Roger: Uhh Scott I noticed you ate your steak with a knife and spoon earlier, so why don't you hold the dummy end of the measuring tape and I'll measure.
Justin: Man, did you see Mr. MacCormack the other day shoveling his driveway!? He was using a fuckin' icepick!
Tristan: Haha yeah, he definitely holds the dummy end