From around 1350, the unit of currency in Spain was called ‘real’. As time went on, Spain introduced a new silver coin with a value of eight reales. This eight-real coin was called ‘piece of eight’. This silver coin had a diameter of about 3.8 cm. This piece of eight coin is what came to be called the Spanish dollar. In Spanish, this piece of eight was called peso de ocho and it was the world’s first currency.
The British government sent those eight-real coins for use in its colonies in Australia, Far East and America. In East Africa coast, the Arabs as well as Portuguese were traders there. The Eastern Bantus were by around 1500 AD on high speed migration. The Bantu language borrowed a few words from Arabic and Portuguese languages to form what is today called the Kiswahili or Swahili.
The Portuguese introduced the piece of eight (peso) to the Arabs and the Arabs introduced it to the Swahili. The Swahili renamed the piece of eight as ‘pesa nane’.
When the British came to East Africa by around 1880, the currency in use then was the piece of eight. The Africans would call it pesa nane. But as soon as the British took over control of East Africa, they introduced a new currency. In the new currency, the piece of eight had a value equivalent to 25 cents. This is the reason 25 cents is called ‘pesa nane’ in East Africa.