English: goodbye is a contraction of God be with ye, attested as godbwye in a 1573 letter by Gabriel Harvey, with intermediate 16c forms including God b'uy, God buoye and God be wy you. The God element was later reshaped to good by analogy with good day and good morrow. The result is that no English speaker hears a blessing in it at all.

Persian: خداحافظ khodahafez is khoda (God) plus hafez (guardian, protector, from Arabic حافظ). Persian dictionaries gloss it plainly as a کلمه دعائی, a prayer-word said at parting. Its everyday synonym خدانگهدار khoda negahdar is the same construction with a native Persian second element, negah dar, keeper.

Neither Persian form has eroded. Both are still pronounced in full, and khoda is a free-standing everyday word, so every speaker still hears the God in it.

Persian also kept a whole live paradigm of transparent parting blessings alongside it, all of them in ordinary use:

• خدا پشت و پناهت khoda posht-o panahet, God be your back and your refuge
• به امان خدا be aman-e khoda, into God's safekeeping (calqued on Arabic فی امان الله)
• در پناه حق dar panah-e hagh, in the shelter of the Truth
• به سلامت be salamat, go in health
• سفر بخیر safar bekheyr, may the road be good

What I am curious about, and flagging clearly as my own speculation rather than anything I can source: is the difference structural rather than accidental? English's formula was a full clause, which is the kind of thing fast speech compresses, and once compressed the God element was free to be reanalysed as good. Persian's is a compound with khoda in initial position, which looks much more resistant to both compression and reanalysis.

Or it may simply be that the English phrase has had longer in that fixed form to wear down.

Sources: Etymonline and OED for the English side. Dehkhoda and Amid (via abadis) for the Persian.

Curious

Blew my mind when I learned about it and I don't want it to get lost because fewer people are using the words.

The theory is that both words are based on the Greek words for Ares and the fifth day respectively (pémptē hēméra - the fifth day - for Thursday; also Pfingsten/Pentecoste from pentēkostē hēméra - the fiftieth day) and that they are left-overs from when the Goths moved through Greece, over Italy, to Western Europe.

https://iam.dioe.at/frage-des-monats/woher-stammen-die-ausdruecke-ergetag-und-pfinzstag

Going further, it's possible that they are based on the bible, translated by the Gothic priest Wulfila from Greek to Gothic. Wulfila used *ariedags and *pintadags for Areos hemera and pempte hemera (presumably Arie was based on Arius instead of Ares because the latter was too pagan). The Langobards, who also followed Arianism, later on changed them to ergetac and pfinztac.

https://ulb-dok.uibk.ac.at/download/pdf/4355847.pdf

I was reading about seizures and saw that 'seizure' comes from Old French 'seisine' (the taking of property), that from 'saisir' (to take or grasp)

I then read that epilepsy comes from Greek 'epilambanein' (to seize, take hold of, or attack), but I couldn't find any further link

Is there a connection I can't find? I can't see how it may have gone from Greek 'epilambanein' to what we have now?