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.)
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When someone says something dumb or out of context
Shayan: says anything
Everyone : Righto Mate