Underwear that has been filled to the brim with vile human excretion such as anal gravy, sweat, pussy juice, ass juice, butt nuggets, anal hot chocolate, and piss.
"Dude I fingered Tiesha last night, and then I smelled my hand and it reeked of stanky britches."
"I can smell yo britches and they're stanky."
"Hey I gotta change my under wear I have stanky britches."