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.