Pesik reisha

In Halacha (Jewish law) a pesik reisha is an action which will definitely cause as a side-effect an action that would be forbidden. The classic 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.

The phrase is short for "pesik reisha velo yomus?" - Will you cut off it's head and it will not die? This refers to somebody who cuts off the head of a chicken so that his child can play with it. (Evidently in the time of the talmud it was common for children to play with chicken heads.) Even though he did not particularly want the chicken to die, since that was the only way he could get the chicken's head, he is considered to have intentionally killed the chicken. As such it would be forbidden to do on Shabbos.