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The fabrication or exaggeration of an anecdote/scenario, or just general bullshitting from an old relative, usually Nan/Grandmother.

Can be rarely used as a derivative of "Gassing" in a general context w/out an OAP involved.
e.g #1
Nan: "Stop leaving that television on standby, you're charging me in excess of £50 a day on electricity bills!"

John: "Oh, behave it's like 14 pence a day. Stop Nan-Gassing."

Granddad: "Have you noticed the evil glare in our neighbour Sally's eyes whenever you make eye contact? She must be a serial killer."

Jason: "No, that's a genuine Nan-Gass, Granddad."
by Rory Doe December 18, 2012
6 1