"A huevo" is one of the many Mexican slang terms that uses the word "heuvo", which literally means "egg", but figuratively refers to testicles. You can use this phrase when somethings cool happens or as a form of agreement. P1: "Quieres ir a la fiesta?" P2: "A huevo!" The exact origins of this phrase are unknown, but some posit that it comes from a misunderstanding of the phrase "a bueno".