Kapo: possibly from the Italian "capo" meaning “boss”.
A prisoner who has been placed in a position of authority over a prisoner unit, a “Kommando”. This could be Jewish or non-Jewish prisoners. They would receive special benefits like proper clothing and a private room.
If they neglected their duties they would be downgraded to ordinary prisoners and be subject to other Kapos.
The Kapo system was designed to turn prisoner against prisoner, as the Kapos had to maintain the favour of their SS overseers.
After World War II, the term was reused as an insult; according to The Jewish Chronicle, it is "the worst insult a Jew can give another Jew"