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. It has been variously interpreted as meaning "big ears", "flop ears", "lop-eared", "floppy", or "fatty".
Flaccus was also a cognomen for a branch of the patricianValerii and others.
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