Penny bun or a penny loaf was a small bread bun or loaf which cost one old penny at the time when there were 240 pence to the pound. A penny loaf was a common size loaf of bread in England regulated by the Assize of Bread Act of 1266. The size of the loaf could vary depending on the prevailing cost of the flour used in the baking. The nursery rhyme London Bridge Is Falling Down has a version which includes the line "Build it up with penny loaves".
The expression 'penny bun' is Cockney rhyming slang for one, sun and son. 'Penny bun' is also the nickname for the cep, or Boletus edulis, an edible basidiomycete mushroom. Native to Europe and North America, it is Europe's second most sought-after fungus after truffles.
'Cockle to a penny bun' is British slang for racing odds of 10 to 1.