What does "yomayo" mean in English?

That word is a Russian exclamation and is pretty versatile. It can be used in horrific, amazing, or happy occions. Unfortunately, I can't give an expression that is exactly like that in English because there kind of isn't an exact one. In Chekov's situation, it can mean something like, "Oh, my gosh!" or "Unbelievable!" or "Awesome!" If you combined all three phrases, you'd just about get the meaning of "yomayo" in that context.