Pesik reisha
In Halacha (Jewish law) a pesik reisha (Lit. cutting off the head) is an action that ordinarily would be permitted but which will definitely cause as an unintended side effect an action that would be forbidden. Therefore, the ordinarily permitted action is itself prohibited. A classic case of a Pesik reisha found in the Talmud[1] 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? This phrase refers to the most classic case of "p'sik reisha", where if a person cuts off the head of a chicken he cannot expect that it won't die.
There is extensive literature and discussion amongst the Acharonim in this matter, especially in the Brisker school of Torah-study.
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