Shirt Wedgie

When your backpack pulls up on the back your shirt as you walk and tucks it between your back and your pack, exposing the full suppleties and curvatures of your behind and, likely, your Transformers underpants.

"Well, I'll see ya later!"

"Hold up, bro. You got a shirt wedgie goin' on. Pull that shit down before someone else sees your buttcrack."