A pet form of John, Johnny is used, with modifying word, to designate a person, especially a man, of the type, group, profession, etc., specified. For example:
– Onion Johnny, also Johnny Onions, was a generic name for an onion-seller from Brittany;
– Johnny Crapaud is used to personify France or the French people, or to designate a typical Frenchman;
– Johnny Foreigner is used to designate a foreigner, or to personify foreign people.
—Cf. also Joe Six-Pack in American English, and Joe Bloggs and Joe Soap in British English.

Likewise, in British English, Johnny Arab, also Johnnie Arab, is an offensive generic name for an Arab man.

For example, the following is from Britain. A national brand stuck in the 1920s, by Michael Bywater, published in The Independent (London, England) of Monday 28^th February 2011:

The underpinnings are in place. Johnny Arab going crackers toppling dictators right and left, while back in Blighty the flannelled fools are at the wicket, the muddied oafs in the goal. The palpably barking Gordon turns out to have been hand in hand with the even more barking Colonel Gaddafi in a dodgy deal to sell wirelesses to Libyan secret policeman. Plus ça change as Chief Flannelled Fool Cameron plods around the desert flogging bombs and handcuffs to anyone with the Riyals to pay.

The earliest occurrences of Johnny Arab and Johnnie Arab that I have found are as follows, in chronological order:

1-: From a Bathonian’s reminiscences of the Siege of Najaf, an engagement between the British Army and Iraqi rebels in the city of Najaf, Iraq, from 23^rd March to 4^th May 1918, during the First World War, published in the Bath Chronicle (Bath, Somerset, England) of Saturday 28^th August 1920:

I have said that Nedjf was a very holy place, and that the Arabs come hundreds, and even thousands, of miles to worship at the shrine of Ali. The various troops ordered to Kufa and Nedjf soon got into position, and a complete cordon was formed round the walls. We were all impatient for the guns to blow down the walls and allow us (infantry) to enter the city and teach Mr. Johnny Arab better manners.

2-: From Awakening of the Holy Land, by A. C. Cummings, special correspondent of the Southam Newspapers, published in the Edmonton Journal (Edmonton, Alberta, Canada) of Thursday 22^nd January 1931—in the following, A. C. Cummings describes a ceremony at the Wailing Wall, in Jerusalem, when the Jewish Sabbath begins:

Psalms and portions of the Book of Lamentations are also read. Mostly the chanting and reading is in ordinary tones such as you hear in the synagogue; but now and then someone in the swaying group will find his emotions too much for him and he will break out into loud wails and even shed tears.
“The women are the worst,” said the stolid British policemen [sic], while I watched the fascinating ceremony. “Often when they come here they have just had a death in the family. They are thinking of that, of course, and they wail and shriek and tear their hair and carry on something awful. But even then they are nothing compared to Johnny Arab. At his religious metings [sic] they sometimes take knives to themselves. It’s a bit beyond me.”

3-: From Palestine Policeman. An Account of Eighteen Dramatic Months in the Palestine Police Force during the great Jew-Arab Troubles (London : Herbert Jenkins Limited, 1939), by the British soldier Roger Courtney (1902-1946?), who used the form johnny-Arab, plural johnny-Arabs—for example:

A man who could bluff his way into the country like that might be the sort of man to bluff some of the johnny-Arabs who were giving the Administration such trouble.
[…]
To those knowing anything of johnny-Arab, everything about them proclaimed their guilt.

4-: From the Derby Evening Telegraph (Derby, Derbyshire, England) of Friday 28^th May 1943:

An Army unit in Tunisia has its own morning and evening newspapers, “Reveille Rag” at breakfast and “Racing Special” at tea time. They are produced by a former “Telegraph” reporter now in North Africa out of material gleaned from English radio news bulletins.
[…] He explains in a letter home, “[…] Even in the most desolate spots they [i.e., the Arabs] now seem to have developed a palate partial to chewing gum and American cigarettes.
“The Yanks are generous wayfarers, and bestow these goods upon all askers, which makes us very popular with Johnny Arab—‘English American very good—Boche no damn good.’”

5-: From The Tribulations of Post-War Soldiering. In Trieste, Palestine, Egypt, India, and Java, the Soldier Has an Unenviable and Often Humiliating Task, by Ferdinand Tuohy, published in The Sphere (London, England) of Saturday 20^th July 1946:

Many weeks before the current trouble the British troops in Palestine had largely retired to a lonely camp existence. Except in rare cases, they had been unable to make headway with the Jews who, obviously acting under a common order, removed the advantages of social life and made our men feel the chill of dark animosity. Not unnaturally, a cold anger gradually overtook the ostracised, who found themselves thinking not unfavourably of “Johnnie Arab.”