means a little book (Latin, libellus). A lampoon, a satire, or any defamatory writings. Originally it meant a plaintiff's statement of his case, which usually “defames” the defendant.
Dost not know that old Mansfield, who writes like the Bible. Says: `The more 'tis a truth, sir, the more 'tis a libel'?
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894