So quiet or soft that you cannot hear it; unable to be heard.
From Latin 'in-' (not) + 'audibilis' (able to be heard, from 'audire' meaning 'to hear'), literally meaning 'not hearable.'
The phrase 'inaudible whisper' sounds like it should be impossible—a whisper you can't even hear—but we use it all the time when someone talks so quietly that their words completely disappear into the air.
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