Prison or street jargon used as a response to "What's up... <used and said in a way of either jokingly feigning indignation, or, intentionally, ambiguously, or actually... provoking a fight> <in combination with body language insinuating offense of some kind or another whether real or imagined>... 'witchu'with you? The response, "Whatchu wanna be up...my niggah? <usually less threatening>....white boy <racial slur meant to provoke a response of some kind or as a test of mettle <or, usually in a more aggressive tone and more threatening body language, with just a slight tilt of the head and arms positioned in such a way as to appear sort of ape like, or, as if carrying a large imaginary basket...>...nigga? Sometimes accompanied by “gettin' in a nigga face” and aggressive posturing. More common in prison, or the streets, specifically, ritually, it is a common way fights that start off posturing verbally sometimes start ramping up to evolve into pushing and or actual fist fighting. It is a form of social posturing and a "dominance alarm"* (*if that is a thing*) It is used —unprovoked— to test the mettle of a perceptively weak inmate. Sometimes the test completely fails if the person being tested is a new arrival, expecting it and, (as a result of his comrades or whomever he is riding with, assure him is the proper response to such a challenge...) 'steals off on**' the aggressor. (**or, in other words, w/o warning, no posturing whatsoever, makes first punch)
Prison dialogue example:

New boot: “What up, bruh?”

Posturing alarmist/tank boss, or soldier:

“I aintcho 'bruh,' bro!

"I ain't no friendly nigga...

... gets all up in new boot's grill, testing his mettle...)

Whatchu wanna be up, white boi?"

New boot:

"Oh, my bad. 'Bro' it iiis!(wham!)"

Other inmates:

"Oooooweeemain, THAT white boi stole off on dat nigga, main."

Guards in pickett, over intercom, barking:

"Rack it up, maggots.... NOW!"
by MoKa$hMoney August 21, 2021
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