"Fair trial" would be an antonym to kangaroo court.

No. (in the US) a "kangaroo court" is a slang expression referring to an unofficial, unlawful, or "vigilante" court.

Kangaroo justice is that which is dispensed in a kangaroo court. A "kangaroo court" refers to sham legal proceedings where the outcome is decided more or less in advance, regardless of witnesses and the appearance of genuine proceedings. Such a court is usually unauthorised and does not observe the usual protocol of a court of law.

Normally this is described as a "Kangaroo Court." The term is first noted as used in Texas, 1883 - where the court proceeded by "Leaps like a Kangaroo."

Kangaroo court