Dumb as a door-knob is no longer a thing..
"Dumb as a doornail" -- a doornail was the nail/plate behind a door knocker. When one knocked on the door, this doornail was abused. Since few people have these door knockers now days, the saying at some point became "door knob" instead of "door nail. However, this saying isn't quite relevant any more. Not when our doors have actually gotten really smart? How smart can a door be? Take a look: Want to know more about smarter home security? Message us with your questions. --