A huevo

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. It's a way of heartily expressing affirmation, as in: 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.

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 fuerzas

A huevo has a secondary use as an adverbial expression equating with a fuerza or a fuerzas. You can use it when talking informally about something that is required, something that must reluctantly be done. Se tiene que ir a huevo = You have to go whether or not you want to. There's really no other option.