A fork of Rural Dictionary
When you personally have made a repair to a machine and feel that it works better just because you worked on it. Even if the part of the machine you worked on would not make it work any better.
Jim: "I replaced the heater core in my car, now it seems faster..." Sam: "I've heard of that before." Jim: "Seems to run smoother too!?" Sam: "That's Mental Horsepower!"
Mental horsepower refers to intelligence in the purest sense of cognitive ability and memory. Those with high MH have the ability to learn math and science easily and trivially.