Selfsame means exactly the same one. That selfsame day, the selfsame man, on the selfsame spot.
It is emphatic where same is plain — selfsame insists there is no other, usually when the coincidence matters. It always comes before the noun and takes the. It was two words in Middle English and closed up over time.
1adjectiveselfsameExactly the same one.
“She was married in the selfsame church where her grandmother had been married.”
selfsame is a quality of identity.
Middle English self same, two words joined over time. The self here means the very one.
Proto-Indo-European
*sem-
Old Norse
samr — same; agreeing, of one mind
Proto-Indo-European
*swé — self
Old English
self
Middle English
self sam
English, today
selfsame
Built from
From Middle English self sam, self same, selve same (“the very same, selfsame”)silfe same, from self (“that specific (person mentioned), herself, himself, itself, themselves”) (from Old English self, ultimately from Proto-Indo-European swé (“self”)) + sam, same (“(adjective) equal, identical; unchanging; referred to earlier, abovenamed, aforementioned; (adverb) again, repeatedly”) (from Old Norse samr (“same; agreeing, of one mind”), ultimately probably from Proto-Indo-European sem- (“one, together”)). The English word is analysable as self + same. 2
Etymology from Wiktionary, CC BY-SA