Flaccus was a Roman cognomen of the plebeian Fulvii, considered one of the most illustrious gentes of the city. Cicero and Pliny state that the family was originally from Tusculum, and that members still lived there in the 1st century.

As usual for cognomina, "Flaccus" was likely originally a nickname, probably of Marcus Fulvius Flaccus, the founder of the family. It has been variously interpreted as meaning "big ears", "flop ears", "floppy", or "fatty".