Many superstitions have been a part of Jewish life through the ages. One is to spit three times in reaction to something especially good or evil. This is done by literally spitting or figuratively by saying “pooh, pooh, pooh.” Over time the saying of “pooh, pooh, pooh” substituted spitting due to its offensive custom. The explanation for ‘three times’ is because the number three is considered mystical. Long ago, people used to deem odd numbers lucky. Considering three was the first odd number after the unit (one), many people judged it as a magical number.
Upon witnessing, listening, or discovering something shocking, it is customary for believers in this superstition to spit to prevent such a tragedy from occurring. Moreover, spitting is traditional at something marvelous to defend against the Evil Eye (spiritual ability to injure a person). It is believed that spitting will protect one from trickery or evil spirits.
Sources: Eisenberg, Ronald L. The JPS Guide to Jewish Traditions. PA: Jewish Publication Society, 2004.
Kolatch, Alfred J. The Jewish Book of Why/The Second Jewish Book of Why. NY: Jonathan David Publishers, 1989.
Wigoder, Geoffrey , Ed. The New Standard Jewish Encyclopedia. NY: Facts on File, 1992.