unutterable

Not utterable; incapable of being physically spoken or voiced; unpronounceable.

Incapable of being articulated or expressed; indescribable, inexpressible.

unutterable anguish

[H]e is Sir / The moſt vnutterable covvard, that ere nature / Bleſt vvith hard ſhoulders, vvhich vvere only giuen him, / To the ruine of baſtinados.

Not allowed to be spoken; taboo, unspeakable.

Hovv oft the humble Scholar came, / And to your Songs he rais'd his Ears / To learn the Unutterable Name, / To view the Eternal Baſe that bears / The Nevv Creations Frame.

Extremely bad or objectionable; unspeakable.

These unutterable people upon the telephone have already ruined my morning's work, so that a little more can hardly matter.

Something which is unutterable (incapable of being physically spoken, incapable of being articulated or expressed, etc.).