What, you egg! (stabbing)

This line from Macbeth, complete with stage direction, is spoken at the zenith of one of the most horrible and brutal scenes in all of Shakespeare. It has always intrigued me, this line. It’s spoken by an assassin (sent by Macbeth, of course) as he kills Macduff’s young son. It’s weird, right (not the killing, that’s pure evil, I mean the words)? What kind of insult is this? What does it mean? The only thing I can guess at is that he’s calling the kid something “worse” that an infant, like a kind of a pre-infant, undeveloped and immature? So that’s bad I guess.