Cockney rhyming slang
Cockney rhyming slang was a dialect used by Cockneys as part of Cockney English. In the slang, a word would be substituted with another word or a phrase that rhymed with it. Leela noted that it was like a code, when she first learnt about it.
Examples included "boat race" which was slang for "face", "Rosy Lee" for "tea", "apples and pears" as an alternative for "stairs", "plates of meat" for "feet", (AUDIO: Dead Men's Tales) and "butcher's hook" which meant "look". (AUDIO: The Lonely Clock, TV: Ascension of the Cybermen) The Tenth Doctor once said that "Beethoven's Fifth" could be rhyming slang for "Mickey Smith". (PROSE: Legends of Camelot)
Ben Jackson occasionally used it, referring to the Doctor as "China" and explaining that it came from "China and plate", meaning "mate". (TV: Power of the Daleks)