5.1 refers to the state you are in when you become really loud after a drinking session. The 5.1 originates from the surround sound speaker system and hence you can hear the person from every angle.
1) Easy mate you on a 5.1
2) Turn it down bruvu reachin 5.1
New kid- You know that 5'1 guy with the muscles? I'm not intimidated but almost, that guy looks tough.
Guy who's been around school- Yea that's JoJo. The other kids call him GI JoJo, I don't mess with him. He's a good guy to make friends with, he's the most popular guy in school.