The old man only says, "Nada," once, and he means it literally. The rest of the "nadas" come from the narration, which seems to be filtered through the old waiter's perspective:
It was all nothing, and a man was nothing, too...Some lived in it and never felt it but he knew it was nada y pues nada y pues nada. Our nada who art in nada nada be thy name thy kingdom nada they will be nada in nada as it is in nada. Give us this nada our daily nada and nada us our nada as we nada our nadas and nada us not into nada but deliver us from nada; pues nada. Hail nothing full of nothing, nothing is with thee...
Some have said this old waiter is the voice of the old man's soul. Maybe it is Hemingway's voice. But "nada" or "nothing" or "nothingness" is what the old man wants to escape by visiting the clean, well-lighted cafe.
Alone, with his nothingness, the old man suffers from insomnia: he cannot rest knowing his life has been lived unfulfilled. French existentialist Jean-Paul Sartre, who was influenced by Hemingway, says, in his famous work Being and Nothingness, that nothingness is total negation, a non-being, something that fails to exist by making choices.
So, the old man, through eyes of the old waiter, has ceased to exist. His life, his relationship with God, his relationships with men and women, his "Lord's Prayer" are all emptied of meaning. All he has now is monotony, routine, sleepless nights.