That cannot or must not be uttered or expressed: "I burned in the unutterable beauty of being alive” ( John Peale Bishop). See Synonyms at unspeakable.
Being such that pronunciation is impossible: unutterable consonant clusters.
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.