The quality or state of being external, foreign, or unrelated; the characteristic of being outside or not belonging to something.
From Latin 'extraneus' (foreign) + English suffix '-ity' (quality/state). Philosophers and linguists developed this abstract noun to describe the property of externality.
When linguists talk about a word's 'extraneity,' they mean how foreign it sounds—like 'kindergarten' in English, borrowed wholesale from German. Some borrowed words feel native after centuries; others keep their extraneity forever, which tells you which cultures impressed us most.
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