ETYM Dutch boom tree, pole, beam, bar. Related to Beam.
1. A deep prolonged loud noise; SYN. roar, roaring, thunder.
2. A pole carrying an overhead microphone projected over a film or tv set; SYN. microphone boom.
3. A state of economic prosperity.
4. A sudden happening that brings very good fortune; SYN. bonanza, goldmine, manna from heaven.
5. Any of various more-or-less horizontal spars or poles used to extend the foot of a sail or for handling cargo or in mooring.
In economics, a period in the trade cycle when the economy is expanding and aggregate demand (total demand for goods and services) is rising quickly. It is characterized by falling or low unemployment but rising inflation.

Long spar extending foot of sail. boom-sail, boom-sheet, etc., sail, sheet, etc., attached to boom.
A long beam projecting from the mast of a derrick to support or guide cargo.

ETYM Colloq. U. S.
1. To make a deep hollow sound; SYN. boom out.
2. To make a resonant sound; as of artillery; SYN. din.
3. To grow stronger; SYN. prosper, thrive, get ahead, flourish, expand.