Boot Rot is a (as far as I know) slang term for the condtion of having one's feet infected by a fungus. It is a level of foot distress worse than athlete's foot but not as bad as trench foot. Often it comes from wearing a sweaty pair of combat boots for several days. My recent encounter came camping at a music festival that was held during a rainstorm in a swamp in Canada. Three days of wearing the same socks and boots and standing around in swamp mud would be a good way to get boot rot.
Can be cured with combined use of antifungal powder and scrubbing with Lava Soap. Smells bad too. Wearing jungle boots can help prevent this from developing.
See also foot rot.