[for someone or something] to be carried overboard (off the deck of a ship) by water. Our chairs washed overboard in the storm. I was afraid that the dog would wash overboard, so I took her below.
wash someone or something overboard
[for water] to flood up and carry someone or something off the deck of a ship into the sea. The high seas washed two of the sailors overboard. The storm washed our chairs overboard.