The Collaborative International Dictionary of English v.0.48:
Boom \Boom\ (b[=oo]m), n. [D. boom tree, pole, beam, bar. See
1. (Naut.) A long pole or spar, run out for the purpose of
extending the bottom of a particular sail; as, the jib
boom, the studding-sail boom, etc.
2. (Mech.) A long spar or beam, projecting from the mast of a
derrick, from the outer end of which the body to be lifted
3. A pole with a conspicuous top, set up to mark the channel
in a river or harbor. [Obs.]
4. (Mil. & Naval) A strong chain cable, or line of spars
bound together, extended across a river or the mouth of a
harbor, to obstruct navigation or passage.
5. (Lumbering) A line of connected floating timbers stretched
across a river, or inclosing an area of water, to keep saw
logs, etc., from floating away.
Boom iron, one of the iron rings on the yards through which
the studding-sail booms traverse.
The booms, that space on the upper deck of a ship between
the foremast and mainmast, where the boats, spare spars,
etc., are stowed. --Totten.