PC Plod

PC Plod was a derogatory term used for police officers. In Perivale, Ace called a policewoman who was arresting the Seventh Doctor "Mrs Plod". (PROSE: Cat's Cradle: Time's Crucible) Investigating a murder committed by an alien in England on 8 December 1926, the Tenth Doctor told Donna Noble that they didn't want PC Plod sticking his nose in. (TV: The Unicorn and the Wasp) Clyde Langer jokingly referred to Captain Tybo as an intergalactic PC Plod. (TV: Prisoner of the Judoon)

Behind the scenes
The term comes from the fictional character PC Plod from Enid Blyton's Noddy books, which is why the Doctor saying "PC Plod" caused Donna to think about Noddy.