Although it most commonly refers to the beer at the tail-end of a beer, it can also refer to someone's leftovers, or sloppy seconds. This also applies to sexual 'sloppy-seconds' by which it refers to someone that the subject has already had sexual intercourse with.
Dude, like I know you think she's hot, but you'd be doing Dan's sloppy seconds... It's his dreggs man! HIS DREGGS!