Overboard means over the side of a boat into the water. Fall overboard, man overboard, throw it overboard.
Man overboard is the standard shout and starts an immediate drill. Throwing something overboard is the literal act and also a figurative one: throw the whole plan overboard. Go overboard means to do something to excess — they went a bit overboard with the decorations — which is much the commoner use on land. Note that board here means the side of a ship, as it does in aboard and starboard.
1adverboverboardOver the side of a boat into the water.
“A huge wave washed him overboard, and the crew threw him a lifebelt.”
2adverbexcessTo excess.
“They went overboard with the catering and fed about half of it to the birds.”
overboard is a position or degree.
The board is the side of a ship, from Old English bord. The same word gives aboard, seaboard and starboard, which was the steering side before rudders moved to the middle.
Middle English
overbord
English, today
overboard
From Middle English overbord, overborde, equivalent to over + board.
Etymology from Wiktionary, CC BY-SA