Zaddik is a word I discovered from two different directions. First, I am fascinated by the Jewish religion and Jewish culture, and learned the word in my reading about and study of Judaism.
Zaddik comes from the Hebrew word tsadik, meaning wise and just. A zaddik is a holy man (not sure if the term is ever applied to women, but I've always seen it used to describe men) who is considered righteous and wise in the knowledge and practice of Judaism. He may or may not be a community leader or rabbi, but he is generally looked up to as an example for others to aspire to.
Within the Hasidic culture that began in the 18th century, zaddik came to mean the leader of a particular group or sect of Hasidic Jews, and it evolved into a hereditary title handed down within families.
The second way I learned this word is as a great word to have in one's Scrabble/Words With Friends arsenal. It uses two high-value letters, Z and K, and in some variant spellings can use up all the letters on your rack as well.
These variant spellings are all legal in Scrabble: ZADDIK, ZADDICK, TZADDIK and TSADDIK. In Hebrew, the ending -im is used to pluralize nouns, so ZADDIKIM, TZADDIKIM, and TSADDIKIM are also valid words (the plural of zaddick is also zaddikim).