In legal use, a libel must be published. Otherwise the damaging statement may be slander, but not libel, although the latter word is often popularly used when “slander” should be employed.
The legal interpretation emphasized the source of the word, for libel, from the Latin libellus, originally meant a “little book.”
Thus a libel, in its early English use as well as the Latin source, denoted written matter, especially matter that was shorter than that constituting a book. The basis of the Latin libellus was liber, which was actually the name for the inner bark of a tree.
This inner bark was found to furnish an excellent surface upon which to write; hence, by transference liber came to denote “book.”
This Latin term is still used in reference to legal books and is the source of library, a collection of books.