Dialog:

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A:  Yo, what up homie? B:  Not much bro.  Just firing off an email to some retard at work.  A:  Is it an important one or can it wait? B:  It’s not important at all.  I just feel like taking this one guy down a peg.  His new job title went straight to his head and now he’s just being a loser to everyone. A:  Why are you wasting your time on that kind of thing? B:  Just for the hell of it really.

What up?:
This is very similar to "what’s up" but even more slang.  It sounds a bit ridiculous but you can use it with friends for fun.  You can answer the phone like this too, if the person calling you is very close.  Never talk like this in a business or formal situation.

Homie: 
This is super slang.   It’s a way of saying "friend" or "buddy" or something like that.  Don’t overuse it, but it’s something you can use maybe once a month.

Bro:
Bro can be short for "brother" but it can also be used to say "close friend". 
If you say, "My bro is coming to visit me next weekend", that probably means you are talking about your real brother.

If you say, "hey, what’s going on bro?" you could just be speaking with a close friend.  You can use this in casual face to face conversations, on the phone, in text messages, and in very casual emails.

Firing off an email:
One common verb to use to say "sending" an email is "firing off".  It’s referring to the fact that you are trying to do it very quickly because you don’t want to waste a bunch of time writing emails.

Eg.  I’ll be ready to leave in about 5 minutes.  I’ve just got to fire off a few quick emails first.
This is very common in slang spoken English.

Retard:
In slang English, a "retard" is a person who is really stupid or annoying.  You are referring to a person you don’t like.  The real meaning is actually someone born with a real mental problem, but it’s usually not used that way in everyday life.  Friends can call each other this just for fun.

I can’t believe you lost your cell phone at the bar again.  You are such a retard.

I’d rather not invite him to the party this weekend.  He always acts like a retard in social situations.

Can it wait?:
This is a common question that means, "is it so urgent and important that you need to do it now?"
If you are doing something but someone has a different idea, they might ask, "can whatever you are doing wait?"  That means, can you do that later?
Here’s an example:
A:  I’d like to finish some work before we go and play basketball.
B:  Can it wait?  It’s going to get dark in a couple hours.  Let’s play first and you can finish your work tonight.

Taking someone down a peg:
If someone is overly confident, you might want to "take them down a peg".  That means you want to lower their ridiculously high confidence to a "normal level".  Someone could be acting super cocky and arrogant.  If you see someone doing this, you might want to insult them in some way to "take them down a peg".
A really hot girl might have a real attitude and think she’s super awesome just because people pay too much attention to her because of her looks.  If you see a girl acting this way, you might want to say something to "take her down a peg".

Job title:
Someone’s "job title" is the actual name of their job that is written on their business card.  When people get a promotion, they usually get a salary raise, more responsibity, and a new job title.
Sometimes people act differently when they have the word "president" or "manager" in their title.  They usually act worse and more annoying!

Went straight to his head:
If something goes straight to someone’s head, it means that it makes them over confident.
Eg.  "Please don’t tell him you think he’s good looking.  It will go straight to his head and he’ll start acting like a real loser".

Being a loser to:
If someone is "being a loser to" someone else, it means they are treating them really poorly.  They are acting like a jerk to the other person and really not being fair or reasonable.
Eg.  "I personally don’t mind that guy, but I can see why a lot of people don’t like him.  For some weird reason he is such a loser to people he doesn’t know well"

For the hell of it:
If you do something just "for the hell of it" it means that you have no real specific reason to do something, you are just doing it for fun.  You are just doing it because you want to. 

Eg.  "Let’s go out to a club tonight just for the hell of it."