Not advertised or made public. Later also specifically: not made available online.
Now chiefly North American outside the specific use. On the overlap between the specific use and unposted, see note at "post".
Mid 17th century. From un- + posted.
Of a letter, package, etc.: not put in the post; not sent by post.
Uncommon in North American English.
Late 17th century (in an earlier sense). From un- + posted.
Not placed or stationed; not appointed to a particular post.
Late 18th century; earliest use found in John Ash (1724–1779), lexicographer and grammarian. From un- + posted.