At first glance, you might be tempted to think this expression has something to do with eggs, because huevo does mean "egg". But that's not the case here.
Like most slang, you can't translate this literally, so let's take a closer look at things.
"A huevo güey" in this context, is an affirmation of agreement. Of course it's a very vulgar way of expressing affirmation, but let's face it, sometimes a simple "sí" just doesn't convey our true level of excitement. Not to mention it's fun to spice things up once in awhile. Here's an example:
¿Vas a ir la fiesta esta noche?
¡A huevo güey!
Are you going to the party tonight?
Pretty simple eh?
Sometimes you might want to mix things up a little with your friends without being so vulgar, so instead of saying "sí" all the time, you can use "simon" or "sip", both of which are very informal and mean "yeah" or "yep" in English.
¡Ya! That's it for today.
¡Hasta la próxima!