I think GUEST,Derpy (16 Jan 14 - 12:13 AM) gave us the correct answer above. It refers to striking (bringing down) the main-sail of a tall ship. In evidence, I offer the following reference found in Google Books:

The mention of the "chanty man" reminds me of those peculiarly nautical songs known as chanteys, which are now going out before the advancing use of steam instead of sail all over the Seven Seas. They were originally the songs the sailors sang when hauling up the anchor or pulling in some sheet, and they generally consisted of a recitative and air—the recitative sung by the chantyman alone, and the air, or chorus, joined in with infinite gusto by all the men. There is, for instance, the big main-sheet chantey, whereof the following is a verse:

The first was the whale, the king of the sea—
   Y'hay, y'ho, blow the man down!
He came up on deck singing "Helm hard a-lee,"
   Give us some time to blow the man down!

Then the chorus—

Blow the man down, bullies, blow the man down!
   Y'hay, y'ho, blow the man down—
He came up on deck singing "Helm hard a-lee,"
   Give us some time to blow the man down!

From Stories Of The Sea by G.H. Northcroft, found in Great Thoughts from Master Minds, A.W. Hall, 1908, p. 788

We are now so far removed from the era of the great sailing ships that we attempt to understand forgotten jargon in terms of our modern understanding. When we consider the purpose of such chanteys, GUEST,Derpy's post is really the only answer that makes sense. Northcroft's reference to Blow the Man Down as a "main-sheet chantey" would seem to support GUEST,Derpy's view directly from a time when the jargon was still current.

      - Mark