rump-cord

[Mark A Mandel]

In rec.music.filk someone wrote, quoting a song learned in summer camp
as a child:

: Mary had a steamboat,
: The steamboat had a bell.
: Mary pulled the rump-cord,
: And blew it all to ... pieces!

Any insights into "rump-cord"? OED Online and MW Online don't have it.
OED Online has things like "rump-strap" used with horses, but nothing
that makes sense in this connection. It might be a mondegreen, but for
what?

-- Mark A. Mandel