"mindless, militaristic patriot," 1878, picked up from the refrain of a music hall song written by G.W. Hunt, and sung by "Gilbert H. MacDermott" (1845-1901), supporting aggressive British policy toward Russia at a time of international tension. ("We don't want to fight, But by Jingo! if we do, We've got the ships, we've got the men, We've got the money too.")
Hunt's patriotic song of 1878, with a swinging tune ... became at Macdermott's instigation the watchword of the popular supporters of England's bellicose policy. The "Daily News" on 11 March 1878 first dubbed the latter 'Jingoes' in derision .... ["Dictionary of National Biography," London, 1912]
As an asseveration, by jingo was in colloquial use from 1690s (high jingo is attested from 1660s), and jingo here is apparently yet another euphemism for Jesus (it translates French Par Dieu), influenced by conjurer's gibberish presto-jingo (attested from 1660s). The frequent suggestion that it somehow derives from Basque Jinkoa "god" is "not impossible" but is "as yet unsupported by evidence" [OED].