Shuck your nan

The etymology of the expression "Shuck your nan". Is based on the common London based insult "Suck your mum", "Shuck your nan" is rooted in the term "to Shuck an oyster" or to open/de-husk, meaning in order to fuck someones nan, she is so old she would need shucking - or prising open.

Which is nice.

"you're a plum mate"
"Yeah? Well Shuck your nan"