This is a reference to the greek word [βο-ῶπις](http://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.04.0057:entry=bow=pis) or bo-opis, which is translated in different places as one of:

 - ox-eyed
 - cow-eyed
 - cow-faced

Homer uses it throughout his works; it is one of his [Homeric Epithets](http://en.wikipedia.org/wiki/Epithets_in_Homer). For example, he uses it in the [Iliad, Book 1, lines 531-567](http://www.poetryintranslation.com/PITBR/Greek/Iliad1.htm):

> ‘Hera’ replied the father of men and gods, ‘do not expect to know all my thoughts: though you are my wife you would find it a burden. Whatever it is right for you to hear, no immortal, no human, shall know before you; but of what I plan without reference to the gods, make no question, do not ask.’
>
> ‘Dread son of Cronos,’ the **ox-eyed** queen replied, ‘what is this? I have never questioned you, nor asked: you have ever peace to think on what you wish. But now my heart fears silver-footed Thetis, daughter of the Old Man of the Sea, has swayed you; for she knelt by you at dawn and clasped your knees. Dare I imagine that you bowed to her, gave her a firm pledge of support for Achilles, and promised slaughter by the Greek ships?’

It is likely Homer chose to give her this nickname as her [patron animal was the cow](http://en.wikipedia.org/wiki/Hera#Emblems).