Both Uses

utter
in
Flora & Ulysses
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She uttered the blood-soaked words again.†
p. 75.2 *

uttered = said (or make a sound) with the voice
"I told your father —" But the sentence remained unfinished, the truth unuttered, because the kitchen door suddenly swung open to reveal Flora's father.†
p. 146.6

unuttered = not said

standard prefix: The prefix "un-" in unuttered means not and reverses the meaning of uttered. This is the same pattern you see in words like unhappy, unknown, and unlucky.

Definitions:

(1) (utter as in: utter stupidity)
complete or total (used as an intensifier--typically when stressing how bad something is)
(2) (utter as in: utter a complaint)
say something or make a sound with the voice
(3) (meaning too rare to warrant focus)
Less commonly, and archaically, utter can mean to let out.