over the side of a ship or boat, esp. into or in the water: to fall overboard.
—Idiom
2.
go overboard, to go to extremes, esp. in regard to approval or disapproval of a person or thing: I think the critics went overboard in panning that new show.
Origin: bef. 1000; ME over bord, OE ofer bord.See over, board
"over the side of a ship," O.E. ofor bord, from bord "the side of a ship." Fig. sense of "excessively, beyond one's means" (esp. in phrase to go overboard) first attested 1931 in Damon Runyon.