Decimus is one of the Roman numeral names used in large Victorian families--in this case for the tenth child. It is now seeing some new life thanks to the cutting-edge fashion for ancient Roman names.
The current interest in Decimus has been bolstered by The Hunger Games, in which many of the male characters have ancient Roman names. Decimus was also the middle name of Maximus, Russell Crowe's "Gladiator" character. Other Latin names such as Nero and Tiberius are also hot on Nameberry right now.
Decimus is a name that was not only given occasionally to nineteenth century babies, but also to several notable literary characters in the works of Wilkie Collins and Anthony Trollope. Decimus Burton was a noted ninetenth century English architect.