【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.