When you jerk off in bed and you are too lazy to care where you cum, you inevitably squirt your juices in your sheets. You then get tired and fall asleep, bam, a week later you've got a bad case of bunk funk.
Guy #1: "Dude, what are those crusty white patches on your sheets?"
Guy #2: "Damn I forgot to clean that shit after I was whacking the weasel."
Guy #1: "That's right bitch, now you've got a rancid case of bunk funk."