Pikey is a pejorative slang term used in the United Kingdom. It's origins are unclear but it is either derived from an Anglo-Romany loan word pikey meaning an "outcast" or "outsider" - in particular a Romany person who has had too much contact with non-Roma people (gorgios) or someone pertaining to be Romany when they are not. Another theory is that it could be derived from the term "turnpike sailor" originally used to denote Irish Travellers or Tinkers. It could be a slang word used by some Romany Gypsies for Irish 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. 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.
Related terms for urban or suburban miscreants can be found in the dictionary entry for "chav". The popularity of these terms has grown since the 1980s, and their usage reflects serious and light-hearted issues arising from changes in British urban life rather than an increased need to characterize the lifestyle of travellers.
Pikey has quite a different meaning from crustie - a word which became popular during the 1990s and refers to a kind of hippie traveller who rejects many aspects of modern society and identifies with the underground festival and protest culture.
The Oxford Slang Dictionary indicates that the original usage dates back to 1847.
Negative British attitudes towards "pikeys" were a running joke in the 2000 Guy Ritchie film Snatch. In this film, lead actor Brad Pitt convincingly learned how to speak the distinctive pikey dialect.
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.