It's when a guy is sitting down and has shorts on and has his legs spread open and his balls happen to be showing. All you say to him is "BumbleBee Tuna"!
When someones shorts rise so high up their legs while sitting (or even standing..somehow) to the point where it is retardly awkward to those who are around this person. Pants as high to a point where you can practically see this persons boxers or balls.