unutterably

adjective

1. not communicable by utterance; unspeakable; beyond expression: unutterable joy.
2. not utterable; not pronounceable: an unutterable foreign word.

adjective

1. incapable of being expressed in words

adj.1580s, from un- (1) “not” + utterable (see utter (v.)). As a noun, from 1788; unutterables as a euphemism for “trousers” is recorded by 1843.