1. Let's go back to work. Really give it to him.
That depends on what the "work" is. If it's a boxing match, for instance, and you say to Boxer A, "really give it to him!", you mean "give Boxer B a thrashing".
2. What does "Hiaya mate" mean?
"Hiya" means "hello", and "mate" means "friend". It's an informal greeting in some dialects of English (British English, for instance, and Australian English). The "mate" doesn't have to be a real "friend": some people would say "hiya, mate!" to a stranger.
3. What does the expression "You're on " mean?
It usually means, "I agree to the course of action you have suggested".
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