The story
From even + Homer + nods (“be momentarily inattentive or inaccurate”), alluding to line 359 of Horace’s Ars Poetica indignor quandoque bonus dormitat Homerus (“I become annoyed when the great Homer is being drowsy”). The English translation Homer nods has become standard following Pope (1709), but is due to Dryden (1677).
Etymology from Wiktionary, CC BY-SA 4.0.