By Jingo or By the Living Jingo. Basque “Jainko,” the Supreme Being. In corroboration of this derivation it may be stated that Edward I. had Basque mountaineers conveyed to England to take part in the conquest of Wales, and the Plantagenets held the Basque provinces in possession. The word was certainly used as a juron long before the Crimean War.
Hey, Jingo! What the deil's the matter? Do mermaids swim in Dartford water?
Dr. Morris, in his Historic Outlines (p. 210 note, says it is St. Gingulph, and Professor Skeat (Notes and Queries. August 25th, 1894, p. 149) is of the same opinion. According to The Times, June 25th 1877, p. 6, col.
1), it is the Persian jang = War, and the juron “By St. Jingo” is about equal to “By Mars.” But the word had originally no connection with our jingoism. It was common enough in the early part of the nineteenth century. Query. A corruption of Jesus, Son of God, thus, Je-n-go.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894