In a way that cannot be heard or is too quiet to be perceived by the human ear.
From Latin 'inaudibilis,' combining 'in-' (not) and 'audibilis' (able to be heard, from 'audire,' to hear). The adverb form adds '-ly' to create the manner description.
The prefix 'in-' in English is wild—it can mean 'not' (inaudible, invisible) OR 'into' (inspire, input), causing endless confusion. Linguists debate whether these should even be considered the same prefix, yet English speakers manage the ambiguity effortlessly without thinking about it.
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