Green’s Dictionary of Slang

(US gay/prison) the man who is first in line in a gang-rape.

1972

B. Rodgers Queens’ Vernacular 155: The first to gain anal entrance, a highly prized honor, is called the welcome wagon. The second in line is sloppy seconds, while the third fucker is bloody thirds (no other slang terms are used after thirds, probably because the prize is such a mess by then).