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4.
Various definitions

1) To murk someone via a cuss/diss/boiment/par
2) To get excited/become excitable about something minute
3) To portray general outlandishness
4) To produce fantastic lyrical performance in a rap/spit.
Example 1:

Person 1: (Laughs) Your shape up is nuts! (Laughs) How can you go to the barbers and come out looking worse than before like Edward scissor hands cut your hair! Par!

Person 2: (Laughs) Allow it, you're GOING IN on him!

Example 2:

Person 1: Jheeeeezeee!! I'm that guy!! I got an A in my GCSE Food technology! Suwoo!!

Person 2: Rah, you're GOING IN, still. These times nobody cares about Food Tech, you ain't Ainsley Harriet.

Example 3:

*Guy walks past wearing hot pants*

Person 1: (Laughs hysterically) What is that bruddah doin' wearing them mad tight pants! He's GOING IN with the hot pants!

Example 4:

*Listening to American Rapper J Cole:*

"Aye nigga I'm the best man, as if my best friend was getting married,
Even with my eye closed I'm something like a visonary,
Paint you a perfect picture nigga this is picturnary,
Man they goin have to put my picture in the dictionary,
Right next to real then put your picture right next to fictionary..."

Person 1: Jheeze, J Cole GOES IN on this track!
by ManuchoGoncalves November 02, 2010
 
1.
To enter into an activity with unparalleled enthusiasm and fervor.
I'm so hungry, I'm going in on this chicken pot pie!

Oooh this is my jam I'm going in on the dance floor!
by #TeamHellaSwag September 16, 2010
 
2.
To do anything considered wild behavior or to freestyle as in rapping
They bout to get a cipher we going in.
by OriGinaL CompLex September 10, 2008
 
3.
to catch wreck on a microphone, as in to enter the recording booth and murder a 16 bar verse or entire song. The goal is to keep it raw, like pornstars in the 70s with no protection.
Interlude: Jadakiss (Ghostface Killah)
Ah-hah! I might gotta take my shirt off (yeah, kid...)
I like that one (uh-huh, go in, go in!)

Weezy before his verse on A Milli: "Mac I'm going in"

by MetalLungies September 15, 2008
 
5.
To make a move on a girl.
(At a club/bar/party)
Person 1: That chick is hot, I'm going in!
by iopjklbmn December 10, 2009
 
6.
Takes place when you feel the spirit of the lord go through and overwhelm you into tears and/or til you start shouting.
"I feel the spirit! I'm going in!"
by AJay_BigBlue August 14, 2009