From Middle English garne, from Old English ġearn.

garn (uncountable)

1. (obsolete) yarn (twisted fibers for weaving)

From go on.

1. (Cockney slang) A response that expresses disbelief or mockery.
   • 1912 (date written), [George] Bernard Shaw, “Pygmalion”, in Androcles and the Lion, Overruled, Pygmalion, London: Constable and Company, published 1916, →OCLC ↗, Act II, page 125 ↗:

     mrs pearce. […] But you dont know anything about her. What about her parents? She may be married. / liza. Garn! / higgins. There! As the girl very properly says, Garn! Married indeed! Dont you know that a woman of that class looks a worn out drudge of fifty a year after shes married?