Brewer's: Onion Pennies
Roman coins dug up at Silchester; so called from one Onion, a giant, who, the country people say, inhabited the buried city. Silchester used to be called by the British Ard-Oncon- i.e. Ardal Onion (the region of Einion or Onion).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894