Unutterable

ineffābilis

(incapable of being articulated or expressed — see also indescribable, inexpressible)

ineffābilis

(not allowed to be spoken — see also taboo, unspeakable)

inēloquibilis

(not utterable; incapable of being physically spoken or voiced — see also unpronounceable)

inēloquāx

(not utterable; incapable of being physically spoken or voiced — see also unpronounceable)

īnfandus

(incapable of being articulated or expressed — see also indescribable, inexpressible)

īnfandus

(extremely bad or objectionable — see also unspeakable)