Go by the board
Finished with, as in thrown overboard.
The board is the side or the decking of a ship. In common with many nautical phrases, 'go by the board' dates back to the 17th century. Most of the early references to this phrase relate to masts of sailing ships that had fallen 'by the board'; for example, John Taylor's Works, 1603:
"In this fight their Reare-Admirals Maine Mast was shot by the boord."
and The London Gazette No. 60/3, 1666:
"Our Main-stay, and our Main Top-Mast came all by the board."
It isn't clear exactly whether the phrase 'go by the board' originated with the meaning 'gone over the side' or 'fallen onto the deck'. The usually definitive Admiral William Henry Smyth gives equivocal meanings in his listing of the term in The Sailor's Word-book: An Alphabetical Digest of Nautical Terms, 1865:
"By the board. Over the ship's side. When a mast is carried away near the deck it is said to go by the board."
The figurative use of the phrase began in the mid 19th century; for example, this early citation, from The Gettysburg Republican Compiler, November 1837:
"Those banks that do not resume speedily will go by the board."
Items which go by the board could be said to be jetsam - see flotsam and jetsam.
See other Nautical Phrases.