The term — describing an irresponsible body that is preprogrammed to disregard justice — goes back at least as far as Aug. 24, 1841, when The Times-Picayune of New Orleans used it in reporting the lynchings of several men.

The Americanism spread to international English, but its full origins are a mystery.

Some speculate that the meaning may be linked to Australian immigrants, or derive from a similarity between a kangaroo’s jumping and a court’s jumping to conclusions.

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Thank you

To Mark Josephson and Eleanor Stanford for the break from the news. Jack Begg, our research manager, wrote today’s Back Story. You can reach the team at briefing@nytimes.com.

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