From Longman Dictionary of Contemporary Englishunutterableun‧ut‧ter‧a‧ble /ʌnˈʌtərəbəl/ adjective literary VERYan unutterable feeling is too extreme to be expressed in words —unutterably adverb
Examples from the Corpus
unutterable• It was a moment of unutterable excitement.• And with unutterable gratitude and joy he put his arms around his love and saw her smile into his eyes and blush.• As they clung together in that unutterable pleasure, he felt that they were defying everything that had persecuted them.• There is an unutterable sadness around Medina del Campo, where I had to change trains for Salamanca. 