ineffable

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").