'Same' is a Viking loan from Old Norse 'samr' — from PIE *sem- (one). It replaced native 'ilca.'
Identical; not different or other; referring to a previously mentioned thing without change.
From Old Norse 'samr' (same, identical), from Proto-Germanic *samaz, from PIE *somHós (same, together), from the root *sem- (one, together, as one). This is one of the clearest Norse loanwords in English — Old English used 'ilca' for 'same,' but the Viking settlers' word replaced it entirely during the Danelaw period. The PIE root *sem- is extraordinarily productive: it produced Latin 'similis' (similar — whence 'similar,' 'simulate,' 'simultaneous'), Latin 'simplex' (single-fold — whence 'simple'), Latin 'semel' (once), Greek 'homos' (same — whence 'homogeneous,' 'homosexual'), Greek 'hama' (together), Sanskrit 'sama' (equal, even), and Old Irish 'samail' (likeness). The root also appears in the English prefix 'same-' and in 'some' (originally 'a
English 'same' is a Viking import — Old English used 'ilca' instead. When Norse settlers and Anglo-Saxons began living side by side in the Danelaw, the Norse word won out, probably because it was easier to use and closer to the shared Germanic root that both peoples recognized.