It originated in a few ways. The one I know is that Tupac used punk-police in a few of his songs, and people mistook it as Popo.
"Runnin' from all these punk police..." -Tupac
He says it fast and it sounded like popo.
Orgin: California late 80's
police officers that patrols certain beaches on bikes wore a vest that said PO in huge blockletters on each of their chest. which means Police Officer. they usually ride around in group of two's.
when you see them coming by. you see the word "PO" "PO" when they stand next to each other.