1.Definition: defying expression or description

"Unutterable contempt"

Related Adjective(s):indefinable, indescribable, ineffable, unspeakable, untellable

2.Definition: too sacred to be uttered

Related Adjective(s):ineffable, unnameable, unspeakable

3.Definition: very difficult to pronounce correctly

"Unutterable consonant clusters"

Related Adjective(s):unpronounceable