Unutterable, inexpressible, unspeakable, ineffable, indescribable, indefinable mean incapable of being told or described.
All are often nothing more than intensives implying an extreme that goes beyond the power of words to express. In its more explicit denotations unutterable can imply such various reasons why the thing so qualified cannot be voiced or spoken, as the greatness of one’s awe.
Inexpressible usually applies to what is so delicate, so immaterial, or so subtle, that there are no words to reveal its true or exact nature.
Unspeakable differs little from unutterable in its explicit meaning. More often, perhaps, unspeakable means too unpleasant, disgusting, or horrible to describe in detail.
Ineffable is a near synonym of inexpressible , but. carries a stronger suggestion of a character that transcends expression because of some elusive quality (as etherealness, spirituality, or ideality).
Indescribable and indefinable may imply to the thing a quality or sometimes the lack of any quality or to the would-be describer or definer a deficiency (as of perceptiveness, understanding, or language) that makes precise description, definition, narration, or explanation impossible.