I have been wondering in my head when is it more appropriate grammatically and more appropriate in terms of the English language to use word selfsame instead of same.
The research that I have done suggests that ‘same’ can be used as an adjective and a pronoun and as an adverb. While selfsame can only be used as an adjective. So comparably it would appear to be only a subset of grammatical cases where its used. That being said, I am having still having trouble imagining a sentence or a set of rules to follow that indicate when you should use selfsame instead of same.
Google define gives this example:
He was standing in the selfsame spot you’re filling now.
But I can easily see ‘same’ replacing ‘selfsame’ without an apparent change in meaning of the sentence.
selfsame ADJECTIVE [ONLY BEFORE NOUN] FORMAL . . .
used for emphasizing that something is exactly the same as another
He asked me the selfsame question.
So you wouldn’t say ‘the colours orange and amber are nearly the selfsame’ or ‘orange is the selfsame as amber’.
You’d use it when adding emphasis:
This is the selfsame room we had when we stayed here 25 years ago!
But probably not for the less dramatic news:
This is the same room we had when we were on our way to the Grand Canyon last week.
I wouldn’t agree that the usage is formal, but I would say it’s not used too often in colloquial speech.