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The skin-tight underwear that comes pre-sewn on a pair of super duper short running shorts.

timing girls: "Holy shit, you're not wearing any underwear!"
super fly runner guy: "I, as a matter of fact, am not. I am wearing runderwear. Look!"
by teeeheeee September 16, 2008
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The undergarment worn on the waist to thigh and pelvic region when running, either for exercise, sports, pursuit or evasion. This undergarment absorbs perspiration and holds the genitals intact. It should not be worn continually after running, and should be washed soon afterwards.
"When I finished jogging at the track today, my runderwear were soaking wet and quite pungent!"
by Joefeather May 15, 2013
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