A Gallus (pl. Galli) was a eunuch priest of the Phrygian goddess Cybele and her consort Attis, whose worship was incorporated into the state religious practices of ancient Rome.
The first Galli arrived in Rome when the Senate officially adopted Cybele as a state goddess in 204 BC. Roman citizens were prohibited from becoming Galli, which meant that all the Galli were orientals or slaves. Under Claudius, this ban was lifted. Eventually Domitian reaffirmed that Roman citizens were forbidden to practice eviratio (castration).
The Galli castrated themselves during an ecstatic celebration called the Dies sanguinis, or "Day of Blood", which took place on March 24. At the same time they put on women's costume, mostly yellow in colour, and a sort of turban, together with pendants and ear-rings. They also wore their hair long, and bleached, and wore heavy make-up. They wandered around with followers, begging for charity, in return for which they were prepared to tell fortunes. On the day of mourning for Attis they ran around wildly and disheveled. They performed dances to the music of pipes and tambourines, and, in an ecstasy, flogged themselves until they bled.
Stephanus Byzantinus said that the name came from King Gallus.Ovid (43 BC - 17 AD) says that the name is derived from the Gallus river in Phrygia. The term Gallus is also a multiple pun in Latin, meaning a Gaul, or a rooster, as well as a castrated priest.
While these efforts at "folk" etymologies were widespread in classical times, it has been suggested that gallu comes from the Sumerian Gal meaning "great" and Lu meaning "man", humans or sexually ambivalent demons that freed Inanna from the underworld. They originally seem to have been consecrated to the god Enki.