"grinding" is drug-world lingo for an informant going onto the street to identify drug dealers, who are then busted - and robbed - by the police.
Mosby provided information about which "hitters" were carrying drugs; King and Murray would "catch and release" the dealers, stripping them of their money and drugs. Then, according to prosecutors and trial testimony, the officers would illegally sell the drugs to Mosby at a discount. He in turn, he said, resold the drugs back out on the streets.
They called it "grinding," Mosby told the jury.
the officers "grinded" before or after their regular shifts, calling it their real work.