A large copper coin used in the Byzantine Empire, or a leather bag used to hold money or goods in ancient times.
From Latin 'follis' meaning a leather bag or bellows. The word originally referred to the bellows-like pouch, then became the name of a coin because coins were often stored in such pouches.
The same word for a leather money bag eventually became the name of an entire currency—it's like if we started calling all dollars 'wallets' and it just stuck permanently.
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