on some (blank) shit

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 “bullsh-t” or another word ending in “sh-t.” in such a case the second “sh-t” would be redundant, so it is omitted, and the saying is simply, “on some bullsh-t.”
“now i ain’t even on no rap sh-t no more.
i’m on some f-ckin’ you up sh-t 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 -ssured that i such an encounter will indeed occur.”
notice the speaker’s cunning usage of a triple negative in the first ‘on some (blank) sh-t’ which is meant to confuse the listener.

“but you on some bullsh-t, n-gg-.
this yo’ last beer.
get the f-ck off my d-ck
and tell yo’ b-tch to bring her -ss 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 sh-t.
i gotta be the best.
i gotta stay fresh.
i represent the west.”
-daz, “on some real sh-t”

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.”