The Spanish settlers of the New World had a word for a low‑lying islet or coral isle. To them it was a cayo. The natives of the West Indies call such an ocean‑washed speck of land a cay. When peoples of different languages cane in contact with each other, they are sure to swap a few of each others words. For instance, we borrowed countless place names from the Indians.
More often than not, these adopted words go through a few changes as they become part of a new language. When the Spanish word cayo was adopted and given to the tail islands of Florida, it became key and we have called them the Florida Keys ever since.