In ancient Rome, the words were use when a Roman general was parading through the streets during a victory triumph. Standing behind the victorious general was his slave, who was tasked to remind the general that though his master was at his peak at the moment, tomorrow he could fall, or be brought down. The servant conveyed this by telling the general that he should remember… Memento mori – Remember, you must die. It's in the imperative.
Out here the LA big shots have no slaves, just the art collective, DethKills.