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I've read that it's somehow connected with the Canon Law, but I'm not sure. I'm really interested in finding the answer.

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3 Answers 3

It comes from Middle English modyr in lawe. As far as I understand it, the term was first used in the 14th or 15th century. The idea behind it is that your mother-in-law has the same rights and duties as your biological mother and is given these rights and duties by the legal pact of marriage.

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oh, thank yoг for information! –  Juls Jun 13 '11 at 20:58
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@Juls if the answer provided by Andrew solves your doubts, you should accept it. :) –  Alenanno Jun 13 '11 at 21:14
    
Additional information: For the reason Andrew gives behind the meaning of the word, the term was also used to refer to one's step-mother. In general, the term thus referred to a woman who was not your biological mother, but who had the legal rights and responsibilities of same. –  KeithS Jun 13 '11 at 21:26
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Your answer is good, but I was thinking that if you could include some reference, especially for the part of the first usage, your answer would be greatly improved! Just a thought, since it's already good. :) –  Alenanno Jun 13 '11 at 23:22

Here's the OED etymology:

A phrase appended to names of relationship, as father, mother, brother, sister, son, etc., to indicate that the relationship is not by nature, but in the eye of the Canon Law, with reference to the degrees of affinity within which marriage is prohibited. These forms can be traced back to the 14th century. Formerly -in-law was also used to designate those relationships which are now expressed by step-, e.g. son-in-law = step-son, father-in-law = step-father; this, though still locally or vulgarly current, is now generally considered a misuse.

Essentially, you cannot marry anyone related to you "in-law" because Canon Law treats them identically to an actual brother, sister, etc.

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Oh, that's actually what i was looking for..can you pls tell me the source of your info? i just need it for my grad paper:) thnx so much! –  Juls Jun 15 '11 at 5:44
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“-in-law.” The Oxford English Dictionary. 2nd ed. 1989. OED Online. Oxford University Press. 15 June 2011 <dictionary.oed.com/>;. –  Simon Jester Jun 15 '11 at 13:48
    
again thank you, you've practically saved my life :) –  Juls Jun 16 '11 at 11:25

The usage of "in-laws" came a long way, from Anglo-Norman:

in-law XIX. sb. use of phr. denoting connection by marriage, e.g. brother-in-law (XIII), father-in-law (XIV); after AN. en ley, OF. en loi (de mariage) ‘in law (of marriage)

It is just to denote that this "brother" is only a brother due to marriage, and is therefore a brother by law, not by birth.

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should that be "by law, not by birth"? –  Ben Voigt Jun 14 '11 at 1:20
    
haha! I'm making mistakes everywhere, and @Ben Voigt is picking them up for me! :) –  Thursagen Jun 14 '11 at 1:20
    
I had nothing better to do while eating pizza. It was even a Canadian-bacon pizza. Coincidence? –  Ben Voigt Jun 14 '11 at 1:23
    
Ironic...Ironic? Is that how I should use ironic? i'm so confused now! –  Thursagen Jun 14 '11 at 1:30
    
Just think, someone eating pizza made out of one of my cousins...Did you answer the question on pizza fractions? –  Thursagen Jun 14 '11 at 1:30

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