An unauthorised halfpenny coin from Genoa, introduced into England via London by Genoese merchants in the 14c. It was somewhat thicker than the English coin; some continued in use till the 16c. It was called a 'galley ha'penny' because the Genoese were known as 'galley-men', being based in Galley Quay on the Thames.
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.