suck ur nan
this means to politely go and suck you grandma.
Person 1: Suck ur nan
Person 2: Oi what you say?
Person 2: Oi what you say?
suck ur nan
The phrase you hear in public schools across Britain.
Boy 1: oi, suck ur mum
Boy 2: nah, suck ur nan bruv
Boy 2: nah, suck ur nan bruv