Original: It is often assumed, without evidence, that Niger derives from the Latin word for "black", niger, but it would have been more likely for the Portuguese explorers who first wrote this name on their maps to have used the Portuguese word, negro, as they did elsewhere in the world.
Simplified: Many people think that it comes from the Latin word for "black", niger, but there is no evidence for this, and Portuguese explorers would probably have used their own word (negro) on their maps.