Its source is a Latin expression meaning "Ninth."
In ancient Rome, the praenomen (first name) was often ordinal -- Primus or Prima for the first child, Secundus or Secunda for the second, and so forth. This name was used to designate the ninth child, the ninth daughter, or a daughter born in the ninth month.
With the modern trend toward smaller families, such usage has not been common since the 19th century. Of the spectrum of Latin ordinal names, only Nona and Quintus (designating the fifth boy) are commonly used. Sextus (''sixth''), Septimus (''seventh'') and Octavius (''eigth'') are also still in occasional use.