Originating from Egypt, Bum crust describes the phaecal matter that is not properly wiped from the rectum after excreting phoeses. Consequently, the remaining phaecal matter hardens and becomes entwined within the Pubic hair to form what is known as "BumCrust". See arse biscuits
"Ah shit, we ran out of bog roll, I'm like a bum crust factory!"
"Uhhhh, there's all bum crust in ma pants, Anyone got Scouring pad?"