originated in california where bycycle mounted cops travel around in sets of two. they wear vests with giant POs written on them. thus the name po-po originated
2. Slang term used within Police forces to denote disgruntled officers who didn't achieve promotion. Stands for "Pissed off, passed over."
Cop 1: (To new recruit) "I'm telling you the job sucks. Look at me, 15 years of service and I still didn't make Sergeant."
Cop 2 : "Don't listen to him son, he's just a disgruntled old Po-Po."