When you chop off a chicken's head, the pressure of the axe triggers all the nerve endings in the neck, causing that little burst of electricity to run down all the nerves leading back to the muscles, to tell them to move. The chicken appears to flap its wings and to run around – even though it's already dead.

Can headless chickens run?

Like any person "running around like a chicken with its head cut off" a headless chicken does, indeed, run around crazily and without much thought—at first.

What does running around like a headless chicken mean?

be running around like a headless chicken

​informal. 1. to be trying to do a lot of things quickly without being sensible or calm about it. Synonyms and related words. To be busy.