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Webster 1913 Edition

Adj.

Capable of being uttered.

Adj.

That may be uttered, pronounced or expressed.

utterable ‎(not comparable)

Capable of being expressed in words, especially audibly.
- 1850, Herman Melville, White Jacket, ch. 92,
  An utterable nausea oppressed me.
- 1917, Jack London, Jerry of the Islands, ch. 20,
  In his vocabulary was no word for "crocodile"; yet in his thought, as potent as any utterable word, was an image of dreadful import.