A woman of unscrupulous moral character.
Lets say a guy dates this girl in college. After a year and a half, she abruptly turns very mean on him and breaks it off. Then, she (who had been a virgin before this relationship, no less) promptly goes and screws two of his fraternity brothers/friends, who also happen to be each others biological brothers (!). She winds up dating the second one for a year or two (!!!) thus qualifying herself as "slunk meat".
It has no real definition, but it's derogatory and can describe a skank or someone that your friend has already fucked, sloppy seconds.
Greg: Hey Tom, how are you? I was thinking, I sure wouldn't mind if you fucked my ex-girlfriend Vicki.
Tom: No way! Do you think that I would fuck that piece of slunkmeat?
Simply, really saggy boobs. The kind that just drop to a bitches knees. Also can have pepperoni sized nipples.
Damn that bitch got them slunk meat titties. The kinda that just sag to da floor