On Saturday 28 October, we commemorate when General Metaxas said Oxi to Mussolini’s ultimatum to enter Greece (1940)...
I guess it's like like the "Nuts" response in the Battle of the Bulge
Looks like it translate to No or Nope.
I suspect that the meaning of the term has a deeper meaning to native Greek speakers, such as Natasha, than it does to native speakers of American English.
I think, kind Sir, that "Nuts" is a very apt description for the idea behind the phrase.