I assume derived from "chinny
chin chin". To point out something said is a lie. A version of the saying for
teenagers.
"Beard on tash with a
little bit of mash" implies the lie is ludicrous. If less ludicrous you can use "Beard", "Beard on" or "Beard on tash".
All sayings must include a chin stroke.
Believe this comes from Portsmouth (Pompey) and surrounding areas of the South of UK.