A kangaroo court is not a legal proceeding in the Australian outback, nor does it involve hopping marsupials. 
It refers to a criminal proceeding that is conducted for show, where the defendant is undoubtedly going to be found guilty.
The earliest use of the term was recorded not in Australia as you might expect, but in Mississippi, around 1850. The term ‘mustang court’ is also found from that time, so it seems likely to have something to do with the wildness of the animals concerned.
The term kangaroo court was unknown in Australia until it was introduced there from America.
It is sometimes assumed that the phrase describes how the defendant will be bounced from the court to the gallows.
The term may have also have arisen from the way a kangaroo court defies the law, just as the kangaroo’s appearance and the way it bounds along seem to defy the laws of nature.
My understanding of a kangaroo court was that it was one that jumped (hence the kangaroo) to conclusions, usually not waiting for any facts or evidence.
That is a lovely theory, Linda, and makes a lot of sense. I have also read that it is because the vacant stare of a kangaroo is like the impartial gaze of the jurors.
My version of a kangaroo court is where an individual’s character (or some “jumped up” charge)is cross examined by a group of people however the individual does not have the benefit of a reply because he is absent. No one in the group has the integrity to stand up for the person.