Loud, deep sounds like thunder, or periods of rapid economic growth and prosperity, or long poles used on ships.
From Dutch 'boom' (tree/pole), possibly from Germanic roots. Originally meant a wooden pole, then extended to the sound a large object makes, then metaphorically to economic expansion. Multiple meanings developed through semantic drift.
The word 'boom and bust' describing economic cycles is such an old phrase that it shows how traders have always understood that good times never last forever!
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