Question

What is a "pipel"? (Holocaust def.)

Answer 1

A "pipel" (assistant) was an young teen boy who was the favorite of a "kapo" (prisoner-boss) in a Nazi concentration camp. 

This implied sexual favors being provided by the youngster by the "kapo" or captain in charge of other prisoners.  The etymology of the term "kapo" is unclear, but it may have derived from the Italian word for "boss" or the French word for "corporal."  At any rate, these "kapos" were Jewish prisoners themselves, who were put in positions of supervision by the Nazis in charge of the camps where Jews were held.  "Pipels" served as their assistants and were used/abused in other ways.  There also were "sonderkommandos" or "special units" of Jewish prisoners who were made to be workers in assisting with tasks such as disposal of bodies of gas chamber victims. 
The entirety of the Holocaust was an awful affair, and placing some of the detainees in positions of authority over others only added to the strain and ugliness of what went on in the camps.  Elie Wiesel's told of camp experiences in the book, Night (1956 in Yiddish, 1960 English translation).  He described how kapos and the pipels they favored typically were cruel and brutal in their attitudes and actions toward other prisoners in the camps.

Answer 2

There are a few different definitions but they basically mean the same thing. A pipel is a boy (around 12-15 and usually attractive) who was favored by an SS man (or Nazi concentration camp detainee) and maintained a relationship with the detainee who had special privileges over other detainees. I hope the makes sense. Wiktionary explains it basically the same way