Black's Law Dictionary
Kangaroo court: Term descriptive of a sham legal proceeding in which a person's rights are totally disregarded and in which the result is a foregone conclusion because of the bias of the court or other tribunal.
*West's Encyclopedia of American Law
Kangaroo Court: [Slang of U.S. origin] An unfair, biased or hasty judicial proceeding that ends in a harsh punishment; an unauthorized trial conducted by individuals who have taken the law into their own hands, such as those put on by vigilantes or prison inmates; a proceeding and its leaders who are considered sham, corrupt, and without regard for the law.
The concept of kangaroo court dates to the early nineteenth century. Scholars trace its origin to the historical practice of itinerant judges on the U.S. frontier. These roving judges were paid on the basis of how many trials they conducted, and in some instances their salary depended on the fines from the defendants they convicted. The term kangaroo court comes from the image of these judges hopping from place to place, guided less by concern for justice than by the desire to wrap up as many trials as the day allowed.
Application by U.S. Supreme Court:
*The term is still in common usage by defendants, writers and scholars critical of a court or trial. The U.S. Supreme Court has also used it. In Re Gault, 387 U.S.1, 87 S. Ct, 18 L. Ed 2d 527 (1967), a case that established that children in juvenile court have the right to due process, the Court reasoned, "Under our Constitution, the condition of being a boy does not justify a kangaroo court." Associate Justice William O. Douglas once wrote, "Where police take matters into their own hands, size victims beat and pound them until they confess, there cannot be the slightest doubt that the police have deprived the victim of a right under the Constitution. It is the right of the accused to be tried by a legally constituted court, not by a kangaroo court".(Williams v. United States, 341 U.S. 97, 71 S. Ct. 576 95 L.Ed. 774 
In Rideau v. Louisiana 373 U.S. 723 (1963) the U.S. Supreme Court used the term kangaroo court in clear and forceful terms to denounce and overturn the conviction and death sentence of Wilbert Rideau for armed robbery, kidnapping and murder flowing from a trial where the local Sheriff televised a 20 minute interrogation and confession by Rideau and then had him arraigned. When counsel appointed for Rideau brought a motion for a change of venue the trial judge denied it. Rideau was convicted and sentenced to death. The Supreme Court reasoned that it was a denial of due process of law to refuse the request for a change of venue after the people of the Parish had been exposed repeatedly and in depth to the spectacle of the petitioner personally confessing in detail to the crimes with which he was later to be charged.
In denouncing the filmed and televised confession, the court makes some instructive points which are of great help in appreciating the meaning of the term kangaroo court. Here are some excerpts from the decision:
"Any subsequent court proceedings in a community so pervasively exposed to such a spectacle could be but a hollow formality. In Brown v. Mississippi, 297 U.S. 278, this Court set aside murder convictions secured in a state trial with all the formalities of fair procedures, based upon "free and voluntary confessions" which in fact had been preceded by grossly brutal kangaroo court proceedings while the defendants were held in jail without counsel. As Chief Justice Hughes wrote in that case, "The state is free to regulate the procedure of its courts in accordance with its own conceptions of policy....[But] it does not follow that it may substitute trial by ordeal." 297 U.S. at 297 U.S. 285 Cf. White v. Texas, 310 U.S. 530. That was almost a generation ago, in an era before the onrush of an electronic age.
The case now before us does not involve physical brutality. The kangaroo court proceedings in this case involved a more subtle but no less real deprivation of due process of law. Under our Constitution's guarantee of due process, a person accused of committing a crime is vouchsafed basic minimal rights. Among these are the right to counsel, the right to plead not guilty and the right to be tried in a courtroom presided over by a judge. Yet, in this case, the people of Calcasieu Parish saw and heard, not once but three times, a "trial" of Rideau in a jail, presided over by a sheriff, where there was no lawyer to advise Rideau of his right to stand mute."
NOTE: *This information is from htttp://legal-dictionary.thefreedictionary.com It is by no means intended to represent an exhaustive discussion of the U.S. Supreme Court's use of the term.