Kadosh

Isaiah 6:3 - "...Holy, holy, holy, is the LORD of hosts..." (Hebrew = "Kadosh, kadosh, kadosh ADONAI Tzvaot...")

The Hebrew word for "holy" is "kadosh" or another variation thereof. It means "set apart, separate, sacred, free from uncleanness.

Leviticus 10:10 says "And that ye may put difference between holy and unholy, and between unclean and clean;"

According to Strong's Concordance, "unholy" means "common." God is holy. This means that He should be revered and not treated as commonplace.