fitness stain
The area of perspiration on an individual's pants/shorts in the butt-crackal or taintal region made visible to the eyes of the surrounding audience; most commonly associated (but not necessarily) with strenuous activities - such as working out.
Hey dude. . . before we go out in public, you gotta do something about that gnarly fitness stain you have goin' on down there! Does that not bother you?