An important statement or admission, made under one's breath; any mumbled exclamation when one is talking to one's self, but accidentally doing so aloud. Example: Some of the most dangerous things in a realtionship are the mutterances that escape your lips late at night, when you are talking but are very close to sleeping.
MUTTERANCE
Meaning of MUTTERANCE in English
Slang English vocab. Английский сленговый словарь. 2012