In all our ancient English books, the commentary is printed in the margin. Hence Shakespeare:
“His face's own margent did quote such amazes.”
Love's Labour's Lost, ii. 1.
“I knew you must be edifled by the margent.” —Hamlet, v. 2.
She ... could pick no meaning ... Writ in the glassy margents of such books.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894