Not included in a list, directory, or official record; for a phone number, not published in a directory.
From the prefix 'un-' (not) plus 'listed' (included in a list). As a compound it became common in the 20th century, especially for phones and addresses kept private.
The term 'unlisted number' became a privacy tool before the internet existed—it's one of those pre-digital words that meant exactly what it said: your number literally wasn't on any printed list in phone books that came with your house.
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