A bunch of fives
The idiom is a slang term and originates in boxing terminology. ‘Boxiana’ (1821), a classic work on the history of boxing by Pierce Egan might contain a reference to the term (but this is unconfirmed, as copies are difficult to find today). In the 1800s, boxing was a common form of fighting. In 1825, the phrase appears in Charles Westmacott’s ‘The English Spy’: ‘…came up to the scratch and floored many a youkel with their bunch of fives.’
The thief threatened me with a bunch of fives if I didn’t hand over my wallet.
Nelson is a bully. He threatens the other kids with a bunch of fives if they disagree with him.
Have you watched the film ‘Million Dollar Baby’? It’s about a boxer but is a beautiful story and is more than about just a bunch of fives.