The outermost layer of excrement (poop). A crust can either be relatively firm, as many people typically encounter, or a variant, depending on one's diet.
Hard crusts generally result is added pain, however leave the least residue. They are also very textured.
Soft crusts feel very smooth upon release, however require a time consuming clean-up. Often times they smell profusely and steam.
I'm on my way to a hot date tonight but forgot to take a shit before I left!
Oh God, Bro, let's hope you don't get a soft crust or you're toast!
Crust is usually referd to in the past tense or in the gerund form, Crusted and Crusting respectively. Crust is itself an amalgamation of the words 'crop' and 'dusting'. What Crusting and crop dusting refer to is the expulsion of one's flatulence over a specific area.
One friend to another:
"Wow, did you catch a wiff of the bar?"
"Ya, I know, I totally crusted the entire bar on my way back to the table"