c. 1300, "formal written statement, a writing of any kind," especially, in civil law, "plaintiff's statement of charges" (mid-14c.); from Old French libelle (fem.) "small book; (legal) charge, claim; writ; written report" (13c.), from Latin libellus "a little book, pamphlet; petition, written accusation, complaint," diminutive of liber "book" (see library). Meaning "false or defamatory statement" is from 1610s. Specific legal sense of "any published or written statement likely to harm a person's reputation" is first attested 1630s.
word-forming element making adjectives from nouns, meaning "having, full of, having to do with, doing, inclined to," from Old French -ous, -eux, from Latin -osus (compare -ose (1)). In chemistry, "having a lower valence than forms expressed in -ic."
<a href="https://www.etymonline.com/word/libelous">Etymology of libelous by etymonline</a>
Harper, D. (n.d.). Etymology of libelous. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/libelous