Keep reading, but "lower the boom" is obviously has a sailing origin...
I am unhappy about the definitions, since the only uses I recall for this phrase (all, I think, U.S.) have implied 'putting a stop to something', 'forcibly or by legal process ending errant behaviour' (cf. the citation). phrases.org has some suggestions along similar lines ( http://www.phrases.org.uk/bulletin_board/24/messages/564.html ) but some of their entries suggest an origin in the boom of a sail, which seems an unlikely procedure to me (one can more readily lower the sail of a boat - which would indeed ordinarily lower the boom - but in that case the phrase would be different. To lower the boom on its own you need to unfasten a particular line, the topping lift, which is not usually very accessible, and whose loosening would allow the boom to descend only a little unless the sail was lowered as well. An ineffectual procedure.) I am more tempted by the boom used to stop things floating down a river, or to stop unauthorised entry into a harbour. OED is unhelpful. Does anyone have any bright ideas ?Ardj 21:29, 22 April 2010 (UTC)
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It makes sense that "lowering the boom on" someone comes from trapping them in the harbor. If you'd been unable to prevent their entry (and perhaps subsequent pillaging of your town), then you could at least prevent their escape. Also, you could trap them in a place where cannon-fire from on shore could do damage to the ship "boomed" in the harbor.