'Grockles' is a pejorative term for tourists, applied especially to tourists from the Midlands or the North of England.
It was first noted in Devon during the 20th Century and has spread over time throughout the West Country, notably to Cornwall where there is a particular resentment against tourists, to please whom quiet villages are infested with grotesque plastic pixies (or piskies, in the Cornish language).
The term is sometimes seen on T-shirts - 'I'm not a grockle, I live here' - but these are almost exclusively worn by grockles, as no self-respecting Cornishman would be seen dead in such a garment.
According to the Oxford English Dictionary, the term was first popularized by characters in the film The System (1962), set in the Devon resort of Torquay during the tourist season. According to research by a local journalist in the mid-1990s, the word originated from a cartoon in the children's comic Dandy entitled 'Danny and his Grockle' (a magical dragon-like creature). A local man, who had had a summer job at a swimming pool as a youngster, said that he had used the term as a nickname for a small elderly lady who was a regular customer one season. During banter in the pub among the summer workers, the term then became generalized as a term for summer visitors.
This appears to have been at or around the summer in which The System was filmed. The OED know of no instances of the word dating from before the release of The System, so although some people swear it is an old West Country dialect word, it seems most likely that this is the true etymology.