nouninformal, derogatory, offensive
1Something regarded as inferior to something else; second choice or position. In later use usually: something which has been previously used or rejected by another person; a cast-off.
Apparently used variously with allusion to either "second" or "second".
2The action or fact of being the second man in a sequence of two (or more) men to have sexual intercourse with the same partner; an act of sexual intercourse of this type, viewed as having less status or appeal than the first act; (also) either of the people involved in such an act of intercourse. Potentially offensive.
With reference to the notion of the first man's semen being present in the passive partner.
3Involvement in a sexual or romantic relationship with a person who is regarded as another's cast-off; (also) someone else's former sexual or romantic partner, viewed as discarded or unwanted.
1950s; earliest use found in Berkshire (Pittsfield, Massachusetts) Evening Eagle. From sloppy + the plural of second.