pet form of masc. proper name John, with -y (3). Used as a contemptuous or humorous designation for some class or group of men from 1670s.
It was the typical name in the North and the Northern armies for a Confederate soldier during the American Civil War, and the Southern soldiers were, collectively Johnnies, generically Johnny Reb. In the Mediterranean, it was a typical name for an Englishman by c. 1800. In the Crimean War it became the typical name among the English for "a Turk" (also Johnny Turk), later it was extended to Arabs; by World War II the Arabs were using Johnny as the typical name for "a British man"). Johnny Crapaud as a derogatory generic name for a Frenchman or France is from 1818.
Johnny-come-lately "a new arrival" first attested 1839. Johnny-on-the-spot is from 1896. Johnny-jump-up as an American English name for the pansy is from 1837. Johnny-cocks, a colloquial name for the early purple orchid (Orchis mascula) is attested from 1883.