The name cougar appears to come from a word used by a German geographer, Georg Marcgrave. In 1648 Marcgrave used the word “cuguacuarana” when referring to the puma, which is the word most often used by scientists. The full Latin (scientific) name is: Puma concolor.
I have deliberately taken the first meaning of the word “cougar” – a reference to a large cat – for obvious reasons
Cuguacuarana is a Guarani word. “Guarani” refers to the Guarani people and their language (Tupi-Guarani language family). They are American Indian people of Paraguay and neighouring regions.
The original meaning of “cuguacuarana” is uncertain. The meaning has been made more uncertain by a typographical error made 300 years ago. It is said that Marcgrave incorrectly transcribed the Tupi Indian word, “sassuarana”, to cuguacuarana. That was a major typo!
Cuguacuarana was shortened to “cuguacuara”. Following that adjustment a French naturalist (an expert in nature history), Georges Buffon, called this wild cat a “cuguar”. Buffon wrote about a number of animals that he had never seen!
From there it is a short adjustment to “cougar”, the name probably most often used today by non-scientists. “Mountain lion is also popular with the public. The evolution of the word over almost 400 years is typical of many words of the English language. The starting point for cougar was a word in an entirely foreign language.
Source: Wild Cats of the World (a great book and highly recommended).