Caius, or Caiius as the elders spelt it, was one of the most common of all Roman praenomina, and was pronounced Gaius, as it is written in St. Paul's mention of "Gaius mine host."  Men indicated it by the initial C; women who bore it, used the same C reversed (Ɔ) on coins or inscriptions.  Valerius, or his imitator, deduces it from gaudium parentum, the parents' joy, but it is more probably from the root-word gai... (History of Christian Names, Yonge, 1884)