1. To be carried over the side of a boat by water, especially a wave or large amount of rain. The boat pitched and rolled in the waves, and three unfortunate souls washed overboard before the storm eventually broke. I'm warning you now—anything not tied down is going to wash overboard.
2. Of water, especially a wave or large amount of rain, to carry someone or something over the side of a boat. In this usage, a noun or pronoun is used between "wash" and "overboard." A huge swell crashed into the boat and nearly washed me overboard. The storm washed several priceless antiques overboard on our way to the museum.
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.
[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.