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Pikey is a pejorative slang term used in the United Kingdom which refers to Gypsies and other travellers. In recent years, the definition has become looser and is sometimes used to refer to a wide section of the (generally urban) underclass of the country, particularly those on whom the lower middle classes look down, or merely a person of any social class who "lives on the cheap". When used to refer to either Irish Travellers or Gypsies, it is considered to have racist connotations; even when it refers to others, many people still consider it to be derogatory and offensive. However, it is also commonly used as a synonym of chav, a word which is not regarded as having racist overtones.
Negative British attitudes towards "pikeys" (in this case meaning Irish Travellers) were a running joke in the 2000 Guy Ritchie film Snatch, making the line "I fucking hate pikeys" (often said with a thick accent) one of many oft-quoted lines amongst the film's fans. For his role in the film, actor Brad Pitt convincingly learned how to speak fluent "pikey" (actually an unintelligible patois used for comic effect).
The American terms "trailer trash" and "white trash" are similar in the condescension and disdain with which they are used, though the stereotypes differ in some particulars.
Note that the plural of "pikey" is "pikeys".
Also note that a pikey is not the same as a chav, although the two terms are often used interchangably in coloquial speech.
The Irish equivalent is "knacker". When using the word "knacker" on tv or radio it is polite to mention that it is being used in the "urban scumbag" sense of the word and not referring to the travelling community.
For more see: Pikey - Wikipedia