Shield, n.: Something to Hide Behind
A shield is an object from behind which one strikes blatantly and maliciously before retreating again behind it to withstand the worst of an assault while he comfortably—and in relative safety—plots when next to strike. See Phalanx
In terms to law enforcement, this translates as a malignant entity that betrays local, state or federal laws at the community’s expense, and then when retaliation rears up, hides behind the hard-earned reputation of his comrades in order to let the majority of the blow be lessened on the character of his colleagues, all the while, preserving his own. See Bitch
"You just hit my car!"
Man staggers out of his car.
"It's okay, I'm a cop."
"I don't care if you're a cop, I know you're not on duty--I can smell the alcohol on you. I'm calling 911."
"You see this?" off-duty cop shows his shield. "This says I can do whatever I want, and if you bother me, I'll call for backup and make something up."
"Bitch, call the cops! I'd love to see you explain how you're drunk ass just hit my car!"
"Who do you think they're going to believe, you or a brother? We take care of our own!"