self-same

The word 'self-same' is a tautological compound formed within Middle English by combining two elements of identical meaning for emphatic effect. 'Self' originates from Proto-Germanic selbaz, carrying the core sense of "self, same", while 'same' derives from Proto-Germanic samaz, meaning "same" or "together". Both roots can be traced further back to the Proto-Indo-European s(w)e-, a reflexive pronoun meaning "oneself", which also spawned the Latin 'se' (as in 'se-cluded') and Greek 'he' (as in 'he-donism'). The fusion of 'self' and 'same' around the 14th century created a reinforced adjective, where 'self' acts as an intensifying prefix to 'same', logically building the modern meaning of "precisely and unequivocally identical" from its constituent parts of "same" + "same".

Structure: self (same) + same (identical) self: From Old English 'self', 'seolf', 'sylf', meaning "same, this, identical". In this compound, it functions as an intensifier reinforcing the meaning of 'same'. same: From Old English 'same', 'seom', meaning "identical, unchanged, alike". It is the core adjective denoting lack of difference.