(n)Any unidentified and undesirable substance, often crusted onto something you'd otherwise like to wear or sit on. Often found on second (or third or fourth) hand couches or thrift store finds.
(v)When someone screws something up, often in a half-assed manner.
Hey, man, I like your new jean-jacket, but what's that gunk on the sleeve?
When I passed out on my friend's couch after the party, I woke up to find my drool had coagulated with some gunk on the cushion.
Todd was supposed to mail the rent, but he gunked it, now we have a late fee. Way, to go, TODD!