Old English self, seolf, sylf "one's own person, -self; own, same," from Proto-Germanic *selbaz (source also of Old Norse sjalfr, Old Frisian self, Dutch zelf, Old High German selb, German selb, selbst, Gothic silba), Proto-Germanic *selbaz "self," from PIE *sel-bho-, suffixed form of root *s(w)e-, pronoun of the third person and reflexive (referring back to the subject of a sentence), also used in forms denoting the speaker's social group, "(we our-)selves" (see idiom).
Trying to define yourself is like trying to bite your own teeth. [attributed to Alan Watts, who did often use the image in this sense in his talks, if not in these exact words]
Its use in compounds to form reflexive pronouns grew out of independent use in Old English. As a noun from early 14c.
perhaps abstracted from Old English swa same "the same as," but more likely from Old Norse same, samr "same," both from Proto-Germanic *samaz "same" (source also of Old Saxon, Old High German, Gothic sama, Old High German samant, German samt "together, with," Gothic samana "together," Dutch zamelen "to collect," German zusammen "together"), from PIE *samos "same," from suffixed form of root *sem- (1) "one; as one, together with."
Old English had lost the pure form of the word; the modern word replaced synonymous ilk. As a pronoun from c. 1300. Colloquial phrase same here as an exclamation of agreement is from 1895. Same difference curious way to say "equal," is attested from 1945.