An inch long crevice found on the derriere, that has escaped the modest covering from trousers.
"Cor, look at her bumcrack, the dirty slapper! You could park a car in that!"
When one has commited a particuarly heinous act of cleverness, nerdiness or Hermione-ness. Stupid boffin.
"Eurgh, what's that smell? It must be Dexter, he's geeked his pants well and proper this time!"