Zombie finger is a slang term for fingers that capacitive touchscreens cannot sense. Capacitive screens rely on the natural electrical conductivity and moisture of human skin to detect a touch. Calluses, extremely dry skin, cold hands, or poor circulation can reduce this conductivity enough that the screen ignores the user's input entirely.
Heard on the job
I can never get this tablet to wake up in the winter, I always get zombie finger until my hands warm up.