To Brown Cap is to accidentally poop while attempting to break wind. The term generally refers to a sold turd as opposed to wet shit (see loose poopies, i.e. shart), that results from an over exerted attempt to fart. It can also reference an 'almost' turd beginning to be released while pushing a difficult fart, that's allowed to slip back into the anus.
Damn, I was digesting that t-bone an hour after dinner on the way to the Mozart festival. I lifted my left cheek, figuring my date wouldn't notice a crisp one, considering the road noise. To my surprise, I started to brown cap and almost lost control of the car. Once there, I double checked my boxers for tell-tale skid marks.