Merriam-Webster's Word of the Day for December 04, 2008 is:

ineffable • \in-EFF-uh-bul\  • adjective
1 *a : incapable of being expressed in words : indescribableb : unspeakable 2 : not to be uttered : taboo

Example sentence:
Ed felt an ineffable joy at the sight of his son walking toward him from the plane.

Did you know?
"Every tone was a testimony against slavery, and a prayer to God for deliverance from chains. The hearing of those wild notes always depressed my spirit, and filled me with ineffable sadness," wrote Frederick Douglass in his autobiography. Reading Douglass's words, it's easy to see that "ineffable" means "indescribable" or "unspeakable." And when we break down the word to its Latin roots, it's easy to see how those meanings came about. "Ineffable" comes from "ineffabilis," which joins the prefix "in-," meaning "not," with the adjective "effabilis," meaning "capable of being expressed." "Effabilis" comes from "effari" ("to speak out"), which in turn comes from "ex-" and "fari" ("to speak").

*Indicates the sense illustrated in the example sentence.