About the formation of the word "euthanasia"

Question

The etymology of euthanasia is pretty straightforward, as this site shows:

Early 17th century (in the sense ‘easy death’): from Greek, from eu ‘well’ + thanatos ‘death’.

What is less straightforward, though, is how come -thanasia was formed off the Greek noun thanatos.

In my opinion, -thanasia should have been the end result if the Greek word for "death" would have been *thanasos: < *thanas- + -ia.

Hence, based on the same model, euthanasia should really have been *euthanatia: *thanatos: < *thanat- + -ia.

Answer 1

One thing that the Oxford Dictionaries entry may not make entirely clear is that the Greek word εὐθανασία was derived much earlier than the 17th century. It is listed in the LSJ, along with seven other words containing the sequence "θανασι". A sound change that applied in a certain stage of Greek changed -ti- to -si- (mentioned in this Linguistics SE answer by TKR). I don't know the details of how these words were derived, but I would guess that this sound change is responsible for the s in euthanasia.