AHD says:
Etymonline says:NOUN: 1. A female having the same parents as another or one parent in common with another. 2. A girl or woman who shares a common ancestry, allegiance, character, or purpose with another or others, specifically: a. A kinswoman. b. A woman fellow member, as of a sorority. c. A fellow woman. d. A close woman friend or companion. e. A fellow African-American woman or girl. f. A woman who advocates, fosters, or takes part in the feminist movement. 3. Informal Used as a form of address for a woman or girl. 4. abbr. Sr. Ecclesiastical a. A member of a religious order of women; a nun. b. Used as a form of address for such a woman, alone or followed by the woman's name. 5. Chiefly British A nurse, especially the head nurse in a ward. 6. One identified as female and closely related to another: “the sisters Death and Night” (Walt Whitman).
ETYMOLOGY: Middle English, from Old Norse systir. See swesor- in Appendix I.
"Gk. eor" looks a little different than the others.O.E. sweostor, swuster, or a Scand. cognate (cf. O.N. systir, Swed. sister, Dan. søster), in either case from P.Gmc. *swestr- (cf. O.S. swestar, O.Fris. swester, M.Du. suster, Du. zuster, O.H.G. swester, Ger. Schwester, Goth. swistar), from PIE *swesor, one of the most persistent and unchanging PIE root words, recognizable in almost every modern I.E. language (cf. Skt. svasar-, Avestan shanhar-, L. soror, O.C.S., Rus. sestra, Lith. sesuo, O.Ir. siur, Welsh chwaer, Gk. eor). Probably from PIE roots *swe- "one's own" + *ser- "woman."
For vowel evolution, see bury. Used of nuns in O.E.; of a woman in general from 1906; of a black woman from 1926; and in the sense of "fellow feminist" from 1912.
