Flaccus was a cognomen of the ancient Roman plebeian family Fulvius, considered one of the most illustrious gentes of the city. Cicero and Pliny the Elder 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. The etymology of "Flaccus" is unknown, but it is similar to other adjectives describing deformities of the body, like brocc(h)us (buck-toothed), lippus (blear-eyed), etc.[1] It has been variously interpreted as meaning "big ears", "flop ears", "lop-eared",[2] "floppy", or "fatty".
Flaccus was also a cognomen for a branch of the patrician Valerii and others.