In Southwest England, the disparaging noun grockle designates a holidaymaker or tourist, a summer visitor.

This noun occurred, for example, in the following from The Daily Telegraph (London, England) of Saturday 13^th April 2013 [page T5, column 7]—Burnham-on-Sea is a coastal town in Somerset, in Southwest England:

Call for ‘grockle’ ban

The term “grockle” should be banned from a seaside town to make it more tourist-friendly, a councillor has suggested. Louise Parkin believes the term’s negative connotations are dampening Burnham-on-Sea’s appeal.
“It’s always used in a very derogatory way for describing people who are vital to Burnham-on-Sea’s economy,” Parkin told a council meeting on Monday.
According to the Oxford English Dictionary a “grockle” is a slightly derogatory term for a “holidaymaker, especially one visiting a resort in Devon or Cornwall”.

Of uncertain origin, grockle seems to have first been used in The System (1964), a British drama film written by Peter Draper (1925-2004), directed by Michael Winner (1935-2013), starring Oliver Reed (1938-1999) and Jane Merrow (born 1941). All the earliest occurrences of this noun that I have found are from reviews of this film, and are as follows:

1-: From a review by Ann Pacey, published in the Daily Herald (London, England) of Friday 4^th September 1964 [page 6, columns 6 & 7]:

If you are the sort of holidaymaker who takes a transistor on to the beach, or appears in braces on the prom, or demands chips with everything, you are a grockle. This contemptuous term is apparently applied by young tearaway residents of seaside towns to the tourists who arrive each year for a fortnight’s relaxation.
First I’ve heard of it, too, but it crops up again and again in The System (general release, X).

2-: From a review by Patrick Gibbs, published in The Daily Telegraph and Morning Post (London, England) of Friday 4^th September 1964 [page 21, column 5]:

The visiting girls are parcelled out as the objects of a brief holiday “romance,” the local girls being left to fend for themselves—until the winter.
The preliminaries of this system are sketched in amusingly to an appropriate background of beat music. So is the background of palm court, pier and prom, where the “grockles,” as the older visitors are disparagingly called, take their pleasure, armed with cameras and transistor radios; the women are caught inelegantly paddling, the men displaying their braces.

3-: From a review by Ray Anker, published in the Liverpool Daily Post (Liverpool, Lancashire, England) of Saturday 5^th September 1964 [page 6, columns 5 & 6]:

IF YOU’RE A GROCKLE (OR GROTTLE) THEN READ ON

Roxham, or as it’s better known, Disillusion-on-Sea, is a British coastal resort situated somewhere in the imagination of a screenwriter named Peter Draper.
The System (general release, September 20) offers you a ninety-minute visit.
But be warned. Never again may you feel that first, fine careless rapture of winkle, rock and kiss-me-quick hat.
For ten to one you’ll discover you’re a grockle (or was it grottle? The film’s sharp heroes talked with such contempt of these beings that it was hard to tell).
Anyway, it’s a terrible thing to be.
Grockles, or grottles, are people who do like to be beside the seaside.
PEOPLE who lie on the sands and expose their braces.
PEOPLE who turn up their trousers and paddle in the briny.
PEOPLE who wear hairy flannels and knee-length shorts and play sand-castles with the kids.
PEOPLE who are fat.
PEOPLE who are thin.
In short, people.
Now the local lads of Roxham aren’t grockles, or grottles, and are very nearly non-people.
They sound as if they reached Roxham by taking a wrong turning off the King’s Road, Chelsea.
They are most assuredly with it. They are cool and clever and flippant and cruel and, oh, ever so cynical. And they do not like to be beside the seaside.
Why don’t they leave?
Because Roxham in the summer supplies them with a ready and tractable supply of young female grockles, or grottles, which gives them something to think about during the winter.
Their leader […] falls for a girl as smooth as he, a cool chick who flips back the flip talk and leaves him aching with a summer love grown cold.
[…]
So Roxham has its occasional pleasures. Even for a grockle. Or grottle.

4-: From a review by Francis Fytton, published in Midland Magazine [page IV, column 6], included in the Birmingham Post & Birmingham Gazette (Birmingham, Warwickshire, England) of Saturday 5^th September 1964:

The “system” is the means by which a group of young men in a fashionable seaside resort (the setting is the Tor Bay coast) intercept young girls travelling down on the train, select them by rota in order to save time-wasting competition and then endeavour to seduce them.
Their leader is Tinker (Oliver Reed), a seaside photographer whose profession enables him to discover the addresses of the more recalcitrant girls.
He and his gang see themselves as outlaws preying upon the tribe—the visitors they call “grockles,” or “grocks.”

The following explanation of the noun grockle was published in the Herald Express (Torquay, Devon, England) of Monday 2^nd July 1984 [page 8, columns 5 & 6]:

SOLVED!
Writer invented grockles for film script

The mystery of where the word “grockle” originated has been solved… we think.
Those who deplore the use of this slang term for holidaymaker can perhaps blame Brixham ¹ playwright Peter Draper.
It was in 1962 that the word was coined. Mr Draper was writing the film script for his play “The System” and with some friends decided to invent a word for tourist.
Not insult
“At the time there was a very famous Swiss clown called Grock,” ² explained 59-year-old Mr Draper of 1 Summer Lane, Brixham.
“A friend of mine remarked one day that holidaymakers must have to be mad as Grock the clown to go to the beach in pouring rain—and that’s how it started.”
“The System” was filmed in Torbay and starred Oliver Reed as a girl-chasing seaside photographer who frequently referred to the “grocks” or “grockles.”
“It was never meant to be an insult but a rather affectionate term. I am quite surprised it stuck and now it is used in various countries all over the world.”
Mr Draper is certain that the term had never been used before “The System” but if any of our readers know differently we would be delighted to hear from them.

¹ Brixham is a coastal town in Devon, in Southwest England.
² Grock (Charles Adrien Wettach – 1880-1959) was a Swiss clown whose blunders with the piano and the violin became proverbial.