unspeakable

Short definition from Wiktionary

Adjective

1. 1
   Incapable of being spoken or uttered
2. 2
   Impossible to speak about.
3. 3
   Unfit or not permitted to be spoken or described.
4. 4
   Extremely bad or objectionable.
5. 5
   That cannot be referenced in source code, due to having no usable name.

Noun

1. 1
   Something that cannot or must not be spoken about.