Cockney rhyming slang
Cockney rhyming slang is a London way of speaking in which a word is replaced by a phrase that rhymes with it, and the rhyming part is then usually dropped, as in "use your loaf" for "use your head" (loaf of bread). Dropping the rhyme is what makes it hard for outsiders: you have to know the full phrase to decode the half you hear.
In practice
A few expressions are part of everyday British English and you will hear them from people who have never thought of themselves as Cockney: "Have a butcher's" (butcher's hook, look), "on my tod" (Tod Sloan, alone), "porkies" (pork pies, lies), "a barnet" (Barnet Fair, hair). Many others survive mainly in jokes, films and souvenir shops. Traditionally, a true Cockney is someone born within the sound of the bells of St Mary-le-Bow church in the City.
Why it matters to newcomers
You do not need to learn rhyming slang, but recognising the half dozen common phrases saves confusion at work and in the pub. If someone says they are "telling porkies", they mean lies; "use your loaf" is an instruction to think. Trying to speak it yourself is best left as a joke.
Related
- Pearly Kings and Queens - another Cockney tradition
- Blue plaque - for the London of history rather than slang
Source: St Mary-le-Bow.