The act of anal sex with a radioactive strap on dildo.
Often linked to increased chances of gastrointestinal cancers due to the radiation, and related to Grangemouth due to the grimy industrial town's refinery plant.
"So I found out that I had bum cancer, and thinking back, it was probably because of that Grangemouth Handshake my ex gave me."
A Grangemouth Munch is a meal consisting of cigarette butts, dirt, shit, cum and broken buckfast bottles all microwaved in the local radioactive pile slapped into a stew