A brown streak left in your underpants. Most common causes of a hashbrown are significant, excessive flatulence that leave remnants on your clothing or a failure to completely wipe after you snap a duece.
Tim: Dude, no amount of bleech is going to get that hashbrown out. Did you forget to wipe again?
Jason: Dude, what are you doing going through my dirty laundry, and why are you holding my underwear? Have you gone brokeback on me?