look up any word, like spook:
 
1.
To be currently engaged in (blank) activity, or behaving in the manner of (blank).

NOTE: The exception to the grammatical syntax of this saying is when the word in the blank is "bullshit" or another word ending in "shit." In such a case the second "shit" would be redundant, so it is omitted, and the saying is simply, "on some bullshit."
"Now I ain't even on no rap shit no more.
I'm on some fuckin' you up shit when I see you.
And I will see you too."
-Eminem, "Girls"

Translation: "From the current point on, creating rap music is not not no longer my top priority. On the contrary, I will primarily concern myself with causing you physical harm the next time we encounter each other. As a footnote, rest assured that I such an encounter will indeed occur."
Notice the speaker's cunning usage of a triple negative in the first 'on some (blank) shit' which is meant to confuse the listener.

"But you on some bullshit, nigga.
This yo' last beer.
Get the fuck off my dick
And tell yo' bitch to bring her ass here."
-D12, "Blow My Buzz"

Translation: "However, my African-American acquaintance, the manner in which you are conducting yourself is not appropriate. Since you appear to be intoxicated, I will see to it that you do not consume another fermented malted barley beverage. I demand that you cease your excessive flattery and encourage your female friend to approach me."

"On some real shit.
I gotta be the best.
I gotta stay fresh.
I represent the West."
-Daz, "On Some Real Shit"

Translation: "That which I am about to disclose, I will disclose in absolute honesty. I must achieve the absolute pinnacle of my profession. I must remain stylish and continue to produce original material. I consider myself to be a personification of the entire Western region of the United States."
by Nicholas D October 21, 2009