A friendly insult used to jibe a close friend that has acted increduously.
Or, an expression of utter disbelief.
Used mostly in the Jewish/Yiddish influenced communities around South East London, post the mass influx of Russian Jews to the East-End. Sometimes adopted into (Cockney Rhyming Slang) by the more traditional elements of the Big Smoke.
"What the hell are you doing with that water pistol in an airport you Moiser Grois?!"
Or, "Moiser Grois! I didn't know that cheese was fermented!"