The word jaguar was first used by Spanish Jesuit missionary Jose de Acosta in his 1604 Natural and Moral History of the Indies, in which he stylized it iaguar (this eventually found its way into English through Portuguese sailors). In subsequent years, spelling varied to include forms like jagvara and jaguara until the English settled on jaguar. Acosta got his word from the local Tupi speakers in South America, who used the term yaguara to mean "beast of prey" (this is related to the Tupi word for "alligator", jacare). Beyond that, we don't know anything due to a lack of written records, but it could be from a hypothesized Je-Tupi-Carib proto-language. The name for the car brand was chosen in 1935, but that wasn't made to be the company's name until 1945, when it was picked to be distinctive and exotic. The abbreviation jag was coined in 1959.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.