1) Ancient roman boy name 2) Ancient Roman praenomina 
Latin, meaning: tenth 
Decimus was a Roman praenomen. It means `tenth`, and was originally used as a name for tenth sons, but it was later used as a generic name. Among those named Decimus were: Decimus Burton was an English architect of the 19th century. ...[praenomen] Decimus (s; classical) is a Latin praenomen, or personal name, usually abbreviated D. Although never especially common, Decimus was used throughout Roman history from the earliest times to the end of the Western Empire and beyond, surviving into modern times. The feminine form is Decima. The name also gave rise to the patronymic...
from which 'decimate' originates - strictly
(in prescriptions) tenth.
tenth.