ETYM Akin to Dutch bolwerk, German bollwerk, Swed. bolwerk, Dan. bolvärk, bulvärk, rampart; akin to German bohle plank, and werk work, defense. Related to Bole stem, and Work, Boulevard.
1. A rampart; a fortification; a bastion or outwork.
2. That which secures against an enemy, or defends from attack; any means of defense or protection.
3. The sides of a ship above the upper deck.
The side of a ship above the deck.

To defend with a bulwark.