Emily Dickinson Lexicon

BOOM, n. [D. boom, a tree, a pole, a beam, a bar, a rafter; Goth. bagms; Ger. baum; Eng. beam; D. boomen, to push forward with a pole; Dan. bom, a rail or bar.]

1. A long pole or spar, run out from various parts of a ship, or other vessel, for the purpose of extending the bottom of particular sails; as, the jib-boom, studding-sail boom, main-boom, square-sail boom, &c. – Mar. Dict.
2. A strong iron chain, fastened to spars, and extended across a river, or the mouth of a harbor, to prevent an enemy's ships from passing.
3. A pole set up as a mark to direct seamen how to keep the channel in shallow water.