I've seen it having Hebrew origins which could imply its an alteration of the word Yahweh which is the name of their God.
There is also Yvon which is a French diminutive of Yves being the name of various French religious figures in Medieval past. This word stems from the Germantic Ivo which links to Yew being a species of tree and associated with archers given that the wood may be used for a bow. This connection though doesn't really fit with Yevon as it is in FFX.
However, there is an alternate take on yew being that of Celtic origins, as it symbolized death and resurrection, something commonly associated with Sin in Final Fantasy X. Yevon is often depicted as the A in the scripture and associated with Z (with Sin) which is analogous with the beginning and end; life and death.
The yew trees needles are poisonous which is part of the reason why they are associated with death. It's possible this could be a form of connection with Sin's scales being similar to that of the needles on a yew tree, but the only connection with poison and Sinscales I saw was a 25% poison resist...
A lot of this is, of course, speculation, but I think there is some merit to the Yvon origins at the very least, especially the association with Life and Death which is a popular theme throughout Final Fantasy X.
I hope this is close enough to what you were looking for.