A friend says she suffers from ineffable sadness. What's ineffable?

Ineffable is an adjective used to describe something that is so overwhelming that it's unspeakable or indescribable.

Aside from those more obvious considerations touching Moby Dick, . . . there was another thought, or rather vague, nameless horror concerning him, which at times by its intensity completely overpowered all the rest; and yet so mystical and well nigh ineffable was it, that I almost despair of putting it in a comprehensible form.

From Herman Melville's Moby-Dick