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1.
n) The resulting rash left between the tops of one's thighs after a prolonged hump (hike). Legs rubbing together, with the addition of an abrasive such as denim or a similar heavy fabric, removes the top layer or two of skin leaving a red, uneven, and sensitive hump rash.

Not always noticed during said hump (due to adrenaline) but becomes immediately obvious once you step into a hot shower.

Can be prevented by wearing compression shorts (Spandex) in the place of your standard undergarments (boxers or briefs).

The difference between a hike and a hump is that a hump involves carrying gear. One can hike with a day pack, a water bottle and a camera. One humps when they are carrying items of greater density such as tents, field rations, ammunition for the mortar and a rocket launcher used for destroying enemy tanks.
"We had a 20 miler yesterday and when I hit the shower I realized I was sporting a harsh case of hump rash."
by Chumbucket November 02, 2006