‘Their purpose from the beginning has been to find me guilty, regardless of the facts. This is the very definition of a kangaroo court.’ So said Boris Johnson, in announcing his departure from parliament, with reference to the Commons Privileges Committee. What have kangaroos got to do with it?

Perhaps a kangaroo court’s essence is not in fact that of finding the accused guilty. That is the work of a show trial: ‘A judicial trial held in public with the intention of influencing or satisfying public opinion, and typically having a predetermined verdict,’ as the Oxford English Dictionary defines it.

A kangaroo court is more like lynch law, named after Charles Lynch, a justice of the peace active in 1780 during a threatened Loyalist uprising in Virginia, when suspects were given summary trials. Lynching developed its own horrible connotations for black Americans in the century after the Civil War.

A kangaroo court held by strikers, mutineers or prisoners essentially lacks legal standing. Since the phrase was coined in the 1840s, it has mostly applied to prisoners ruling on the distribution of the property of a new convict.

But why kangaroo? It clearly originated in America, not Australia. An early source is A Stray Yankee in Texas (1853) by Philip Paxton. What he describes is actually an off-duty lawyers’ game of a mock trial, in a log cabin with whiskey consumed. ‘I do not ever remember to have laughed so long or so heartily before or since,’ declares the narrator. He also calls it a Mestang court, a version of mustang, itself conflating two Spanish words for ‘a stray’.

For the invocation of kangaroos, plenty of explanations have been made up. It is unrelated to Australian diggers in the California gold rush. The admirable Michael Quinion on his World Wide Words website points to an example of the phrase from 1849 in Mississippi, far from the goldfields. So on kangaroo court, the jury’s still out.