Latin word seu comes from Latin si (If, supposing that.), Proto-Indo-European *h₂ew (Again. Away from, off.)
si (Latin)
If, supposing that.
*h₂ew (Proto-Indo-European)
Again. Away from, off.
*h₂ewti (Proto-Indo-European)
aut (Latin)
Or (exclusive or).
seu (Latin)
Either... or... (seu... seu...). Or.