[1] The praenomen Septimus was quite rare at Rome, but it seems to have been more popular in rural Italy.
Chase cites two inscriptions in which it occurs after the nomen of a woman, in the place usually occupied by the cognomen.
However, Septimus is not otherwise attested as a cognomen in either family, suggesting that the order of names was reversed, and that the praenomen was used by the Aebutii and Casperii.
[2][3][4][5] Septimus is the Latin word for seventh, and the name belongs to the same class as the masculine praenomina Quintus, Sextus, Octavius, Nonus, and Decimus, as well as the feminine names Prima, Secunda, Tertia, Quarta, Quinta, Sexta, Octavia, Nona, and Decima.
This explanation does not seem to account for the relative scarcity of Septimus, Octavius, or Nonus; but because parents were generally free to choose any name they wished, irrespective of its meaning, it may have been used for either reason.