early 15c., "series or collection of comments," from Medieval Latin commentarius "notebook, annotation; diary, memoir," noun use of adjective, "relating to comments," from Late Latin commentum ""comment, interpretation" (see comment (n.)). Perhaps the Medieval Latin noun is short for volumen commentarium. Especially "explanation of difficult and obscure passages in a book or other writing" (1530s). Meaning "historical narrative" is from c. 1600. Meaning "description of some public event" is from 1927.