Can anyone tell me why sister-in-law is "BELLE-sœur"? I am curious about the derivation of the term. Why is a sister-in-law a good/beautiful person?

  • 1
    This extends to the father/mother-in-law and the answer might also be interesting for stepfather/mother which have the same translation (beau-père / belle-mère).
    – Chop
    Feb 16, 2016 at 9:07
  • Because you don't want your wife (it might be her sister), your brother (it might be his wife) or your father (in the case of your step mother) to punch you. Everything else has been said.
    – MakorDal
    Feb 22, 2016 at 8:55

2 Answers 2


On parle de la première attestation écrite de belle-sœur en français dans son sens courant (par alliance) en 1423, dans un traité entre Frédéric d'Autriche et Catherine de Bourgogne, l'épouse de son frère :

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Histoire générale et particulière de Bourgogne, avec des notes, des dissertations et les preuves justificatives. - Dijon, de Fay 1739-81, t.3, CCXXVIII, p. CCXXIX

Bel avait déjà été utilisé comme épithète de courtoisie (par exemple bel sire dans la vie de saint Alexis, au 11e) et c'est ainsi que le TLFi explique la composition dans belle-sœur. Le Dictionnaire historique de la langue française (dir. A. Rey, ed. Le Robert) dit que c'était employé au 12e comme terme d'affection pour « chère soeur » ; avant d'employer belle-soeur pour la « soeur du mari » (1423, voir aussi FEW soror, bellus), on avait sororge/serorge au 13e pour désigner la même chose (entre autres). On peut noter que certains emplois anciens de l'élément belle ou sœur peuvent s'appliquer aux deux genres ou bénéficier d'extensions de sens ou d'usages particuliers, et il peut y avoir, selon le contexte, concurrence entre l'orthographe de soror/soeur par exemple, et des mots distincts basés sur les racines senior et socer.

This was not about the beauty meaning but rather used originally as a courtesy (see Larousse 5. dans des appellations) akin to something like (my) dear/good sister. The contemporary meaning (sister-in-law) is attested by 1423.


Beau and belle are originally coming from the Middle Ages. There were at this time specific terms to describe the son-in-law (fillastre), father-in-law (parastre), mother-in-law (marastre). Similarly, for brother-in-law and sister-in-law, serorge was the analogous term. However, when you did like someone from your family, you were usually using the biaus and bele words (which became beau and belle) to refer to those people, for instance bele suer for the sister-in-law.

The original end of those words, astre, became pejorative. Since that time, it has been replaced with beau and belle to refer to those people, giving beau-frère, beau-père, ...

As the language evolved, serorge was also replaced, and led to belle soeur.

Source : Langue-fr.net

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    A fillastre (fillâtre) was a son-in-law, not a sister-in-law. There is no evidence suorastre / soeurâtre ever existed. On the other hand, sororge was used for both brother and sister-in-law.
    – jlliagre
    Feb 15, 2016 at 23:32
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    The following is meant as a bad joke and a gratuitous slam of my wonderful French mother-in-law, but I notice you left out the modern version of “marastre” in your closing sentence. Could that be because you sometimes call yours what I often call mine (belle-doche), which is probably no better than calling her “marastre”? (Regardless, +1 !)
    – Papa Poule
    Feb 15, 2016 at 23:52
  • @jilliagre You're indeed right, my apologies. I've corrected my answer. (Ah ah, good one Papa Poule).
    – Izuka
    Feb 16, 2016 at 7:30
  • Good to mention that despite its very pejorative connotation, marâtre can still be used to make the distinction between "the new wife of your father" and "the mother of your wife" , who both are your "belle-mère" if you don't use marâtre. It also seems that legally, and probably for the same reason, parâtre is used for "the new husband of your mother", but I hardly ever heard it myself.
    – Laurent S.
    Feb 16, 2016 at 9:40
  • @Isuka I am not sure what you mean by "when you did like someone from your family". Do you mean that as comethapaxj'ajax said it was an endearment applied to a biological family member? Feb 18, 2016 at 19:02

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