by jingo!, Informal. (an exclamation used to emphasize the truth or importance of a foregoing statement, or to express astonishment, approval, etc.):
I know you can do it, by jingo!
1660-70; orig. conjurer's call hey jingo appear! come forth! (opposed to hey presto hasten away!), taken into general use in the phrase by Jingo, euphemism for by God; chauvinistic sense from by Jingo in political song supporting use of British forces against Russia in 1878
"mindless, gung-ho patriot," 1878, picked up from the refrain of a music hall song written by G.W. Hunt 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.") As an asseveration, it was in colloquial use since 1694, and is apparently yet another euphemism for Jesus, influenced by conjurer's gibberish presto-jingo (1670). The suggestion that it somehow derives from Basque Jinko "god" is "not impossible," but "as yet unsupported by evidence" [OED].