The interjection ¡A huevo! — a borderline obscene expression due to its association with huevo = ball (testicle) — can be useful when you’re shooting the breeze among pals. Think of it as a way of heartily expressing affirmation along the lines of You bet your ass! Here is an example from a movie:
— ¿Ya copiaste los apuntes? Have you copied the notes yet?
— A huevo. Yep. That’s right, I have.
— ¡Qué rápido! Wow, that was fast.
Alley, Mexico City
You can think of a huevo as an obscene synonym for Simón = (sí). An euphemistic version is ¡A Wilson!
Often the idea of a huevo is closer to Ain’t that the truth! or That’s for sure! Use it to express strong agreement with something just said, not necessarily in response to a question. Somebody just said that América is the best team ever? You chime in with ¡A huevo! Fuck, yeah!
A huevo has a secondary meaning that took me a while to notice. It’s an adverbial expression that basically equates with a fuerza(s). You use it when talking about something that is required, logically or otherwise. Se tiene que ir a huevo = You have to go whether or not you want to. There’s really no other possibility.