When one claims to have accomplished something that is utterly impossible this phrase should be said in response. It is usually pronounced in a sarcastic tone so the other person is informed of the person's disbelief of their claim.
Christian: "I hooked up with Georgie last night."

Joel: "Righto mate."

(i.e Georgie is mint and Christian could not have possibly got with her.)
by Mmmmgeorge! December 6, 2010