kangaroo court
US /ˌkæŋ.ɡəˈruː ˌkɔːrt/
UK /ˌkæŋ.ɡəˈruː ˌkɔːrt/

1.
an unofficial court held by a group of people, especially in a prison, in order to punish someone for breaking rules
:
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The prisoners held a kangaroo court to decide the fate of the informant.
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He was subjected to a kangaroo court where he had no chance to defend himself.
2.
a court that ignores due process and legal rights, often reaching a predetermined verdict
:
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The defendant claimed the entire trial was a kangaroo court.
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Justice cannot be served in a kangaroo court.