In Halacha (Jewish law) a pesik reisha (Lit. cutting off the head) is a permitted action which will definitely cause as a side effect an action that would be forbidden. The classical case of a Pesik reisha found in the Talmud is opening a door next to a candle on Shabbos (the Jewish Sabbath) where the candle will definitely be blown out.
The opening of the door is permitted, but the extinguishing of the fire is prohibited. A practical case of pesik reisha is the dragging of a heavy chair over soft earth which will definitely result in furrows which constitute the melacha (action) of plowing. As a result, it is forbidden according to Halacha for a Jew to drag a chair over soft earth on Shabbos, which will necessarily result in the creation of furrows (one of the 39 melachot)
The phrase is short for p'sik reisha ve'lo yamut? - Will you cut off its head and it will not die?
There is extensive literature and discussion amongst the Acharonim in this matter, especially in the Brisker school of Torah-study.