Apparently inspired by the name of an Egyptian god. According to Wikipedia, Sia (the god) was created from the blood spilled as Atum (another Egyptian deity) cut his own penis.

There have been some arguments about Sia's correct pronunciation. Most people seem to pronounce like "See-a", hence jokes like "See ya" on the forum while the developers insist it should be pronounced "Say-a" like in Japanese "sayōnara" ("goodbye").

So, yeah, funny I can agree with. Catchy too.