The meaning of memento mori tattoo is to cherish life. The most accurate translation of Latin "memento mori" is "remember that you will die." I have read many foreign commentaries on this sentence, saying that it emphasizes the commonness and insignificance of an individual's life. No matter how much property and power he has, he will die.
Memento mori makes people remember death, because people will die, so we should cherish every day of life; While "unknown life, how can we know death" emphasizes that it is impossible for living people to understand death, it also seems to emphasize the meaning of cherishing life.
Source: @__polya_kolotit__