Knee knocker
1) A term used by sailors for the jamb of a water-tight door. A water-tight door's opening is about 20 inches above walking surface.

2) A term used by hot-rod enthusiasts for after market items mounted to the bottom of a dashboard assembly. (ie. Tachometers, Nitrous Oxide switches, etc.)
If you can't lift a swab bucket over a knee knocker, you have no reason to be on my ship.