During the time of wooden ships, sailors often referred to their sailing vessel as “the Boards.”
We still use their language when we board a ship or are on board as part of a crew.
Outboard is outside the boat, while inboard is inside.
When a sailing ship’s mast was broken by enemy cannon or in a storm and couldn’t be salvaged, the captain would order the ropes holding it to be cut, letting it drift away or “go by the board.”
We often say that something lost has “gone by the board”.