Kangaroo court definition

First attested 1853 in a Texas context, possibly from the notion of proceeding by leaps or because the kangaroo's appearance defies nature.

A judicial or quasi-judicial proceeding, or a group which conducts such proceedings, which is without proper authority, abusive, or otherwise unjust.

1856, Alfred W. Arrington, The Rangers and Regulators of the Tanaha, or, Life Among the Lawless: A tale of the republic of Texas., R.M. DeWitt (New York), p. 247,

Sol Tuttle interposed, "Don't you let nobody hev any witnesses in this kangaroo court?"
1933, "The New Pictures: M by Fritz Lang," Time, 10 Apr.,

They take him off to face their kangaroo court in the cellar of a deserted brewery.
2003, David Glazier, "Kangaroo Court or Competent Tribunal?: Judging the 21st Century Military Commission," Virginia Law Review, vol. 89, no. 8, p. 2093,

A military commission trial falling short of the full UCMJ standard is all but certain to be pejoratively judged as a "kangaroo court" in the court of public opinion.