BULWARK (a word probably of Scandinavian origin, from bol or bole, a tree-trunk, and werk, work, in Ger. Bollwerk, which has also been derived from an old German bolen, to throw, and so a machine for throwing missiles), a barricade of beams, earth, etc., a work in 15th and 16th century fortifications designed to mount artillery (see Boulevard). On board ship the term is used of the woodwork running round the ship above the level of the deck. Figuratively it means anything serving as a defence.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)