Derivative of "keep it gangsta". The shortened "keep it" is usually said in either a quick burst (not unlike Sagat from Street Fighter II when he yells "Tiger!"), an exaggeratedly high pitch/white voice, or a low and hushed half-serious voice to emphasize the "solemn" trust given to the other person to "keep it (gangsta)" despite all the opposition one may face.
This phrase's originated as a satirical jab at white people who try to "keep it gangsta" as well as those in the genuine thug subculture who are sincerely concerned about "keeping it gangsta".
K-Fed: "Whatcha think 'bout my new CD, yo?!! Yeeea!"
Travis: "Keep it Mr. Spears!"