The story
From Latin in occipitio quoque habet oculos ("she has eyes even on the back of her head"), from Plautus, Aulularia (The Pot of Gold), act 1, scene 1.
Etymology from Wiktionary, CC BY-SA 4.0.
have eyes in the back of one's head
Today it means: To be particularly, especially uncannily, observant, as if able to see in all directions at once. Full definition →