The Sidecar is a direct descendant of the Brandy Crusta, a long-forgotten New Orleans drink that has enjoyed something of a comeback in the last couple of years. How it got its name is a source of debate: Both a French and English bar claim to have invented the combination of cognac, Cointreau and lemon juice for a customer who arrived at the location in the sidecar of a motorcycle.
Bar veteran Dale DeGroff, however, says the drink’s name references the mixture that’s left in the shaker after straining and served in a shot glass on the side. This little treat is called, that’s right, a sidecar

Référence : https://www.liquor.com/recipes/sidecar/