The gens Pacidia was an obscure plebeian or patrician family at ancient Rome. Few members of this gens are mentioned by the historians, but a number are known from inscriptions. The most notable may have been the two Pacidii who were commanders in the army of Gnaeus Pompeius Magnus during the Civil War.[1]
The nomen Pacidius belongs to a class of gentilicia formed from other names, using the suffix -idius. Normally such names would be formed from names ending in -idus, but in this case no corresponding name Pacidus is known, and the suffix was chosen arbitrarily without a direct morphological relationship to the original name.[2] The root of the nomen is probably the Oscan praenomen Paccius, which was also used as a gentile name. Pacidius would therefore be cognate with the nomen Paccius, and probably also with Pacilius.[3] If the name is derived from the Oscan praenomen Paccius, then the Paccii would be an Oscan family, perhaps of Samnite origin. This seems quite probable, given the fact that most of the inscriptions bearing this name are concentrated in and around Samnium. However, in at least some cases it is likely that Pacidius is instead an orthographic variation of Placidius, derived from the Latin cognomen Placidus, "peaceful".[2]
Despite the relatively small number of Pacidii known, it seems that they favoured the praenomina Publius and Gaius, and perhaps also Lucius, three of the most common names throughout Roman history. They also used Numerius, which although relatively uncommon at Rome, was frequently used by Oscan-speaking peoples, such as the Sabines and Samnites.