When a prisoner--who is sentenced to die--walks the corridor leading to the room wherein his/her life will be ended, by gas or injection, the prisoner is said to be a dead man walking by the prisoners watching because (a) it announces that the person walking is about to die, and (b) to walk (freely) means, to be free, to get out of prison. The prisoner is 'walking', getting out of prison, but not alive. S/he is a dead man walking.Originally Posted by www_dba
now means, at this moment, right now.
by now means, up to this moment, before this moment.
EX: She should have called by now. (i.e., by this time)
EX: Call her (right) now.