The story
Named after the Parisian amateur Charles Amar /ʃaʁl a.maʁ/ (who played it in the 1930s), probably by Savielly Tartakower, who also suggested to name it the Ammonia Opening due to the algebraic notation resembling NH₃ for ammonia (compare the Durkin Opening/the Durkin Attack also being named the Sodium Attack). As a joke, the chess player Tim Harding suggested that Amar is an acronym (or rather, a backronym in this case) for Absolutely mad and ridiculous.
Etymology from Wiktionary, CC BY-SA 4.0.