Codswallop
Etymology
First recorded surprisingly recently, in mid-twentieth-century Britain, and its origin is genuinely unknown. A much-repeated folk etymology credits Hiram Codd, whose 1870s soft-drink bottle supposedly gave "Codd’s wallop" ("wallop" being slang for beer) the sense of weak, worthless drink. Etymologists find no evidence for this and treat it as an appealing but unproven story.
noun · /KODZ-wol-up/
Nonsense; foolish talk not worth taking seriously.
What it names
A very British, faintly comic way to dismiss a claim as rubbish — lighter and more scornful-fun than "nonsense."
Example
The idea that reading in dim light ruins your eyes is complete codswallop.
Related idea
Poppycock is nonsense too, but has a traceable Dutch origin; codswallop’s pedigree is a mystery dressed up in a charming legend.