Selfsame means exactly the same, identical, or the very one being discussed. When you say “the selfsame person,” you're emphasizing that it's that exact person, with no possibility of confusion or substitution.

You might use this word when coincidences seem remarkable. Imagine discovering that your new classmate attended the selfsame summer camp you went to three years ago, in the selfsame cabin, with the selfsame counselor. Each “selfsame” drives home how unlikely and precise the match is.

The word adds emphasis that plain “same” doesn't quite capture. If you say “we read the same book,” that's ordinary. But if you say “we were reading the selfsame book at the selfsame moment,” you're marveling at the exactness of the coincidence.

Writers often use selfsame when they want readers to pay special attention to an identity or connection. In The Secret Garden, Frances Hodgson Burnett writes about “the selfsame moor” to emphasize that it's the exact moor, not just any similar landscape. The word has an old-fashioned, slightly formal sound, which makes it perfect for moments when you want to underscore that something is precisely, unmistakably, exactly the same thing.