Does the いは in 或いは come from any term, and if so, how is it/was it used?

Question

I was curious about いは in 或いは because ある is in its plain form however I can't find anything on it. I don't have any physical dictionary, either nor knowledge of if this is archaic Japanese

Answer 1

Surprisingly, this does seem to contain the 連体形 of ある, followed by an "adverbial particle" (副助詞) い which the etymology doesn't define, and the regular topic particle は.

The etymology explains that the original sense is ある人は ("a certain person"),ある場合は ("in a certain situation"), which is reflected in the first definition:

同類の事柄を列挙していろいろな場合のあることを表す
Demonstrates the fact that there exists a number of situations, by enumerating a sequence of related possibilities

For this first definition, the dictionary gives the example:

或いは歌をうたい、或いは笛を吹く
Either singing a song, or playing a flute

So together with the explanation ある場合は "In a certain situation", you can see that the original definition would have meant something like "In one case, singing a song; in another case, playing the flute".

This first definition was generalised to the more abstract notion of "X or Y".