You are translating the specific words "sloppy" and "seconds".
Sloppy in this instance means messy.
Seconds means something that comes after something else - e.g. Bill was first, Ted was second. Bill will say Ted was second to him.
So the expression is meant to signify that Bill is saying Ted is getting his "sloppy seconds", he is getting the woman AFTER Bill has been with her and presumably Bill has left the area (womans orifices) "messy"!
The expression is explained in prior posts above.