In essence, a Mustard Patch is the remains of an "Aristocratic-style" proportion drunken affair in which a moist and viscous collection of various yellow bodliy fluids (both male and female) is deposited.
"Wow, Jerry. We were so drunk last night that I woke up with a Mustard Patch and I couldn't shower 'cause my folks showed up and I was late for work. I spent the whole day apologizing."
by Hexicon June 24, 2006