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See more synonyms for in-law on Thesaurus.com
  1. a relative by marriage.
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Origin of in-law

1890–95; back formation from mother-in-law, brother-in-law, etc.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Related Words

father, sibling, mother, cousin, folk, uncle, niece, aunt, clan, tribe, group, people, house, household, agnate, cognate, connection, blood, folks, relation

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British Dictionary definitions for in-law


  1. a relative by marriage
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  1. (postpositive; in combination) related by marriagea father-in-law
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Word Origin

C19: back formation from father-in-law, etc
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for in-law


1894, "anyone of a relationship not natural," abstracted from father-in-law, etc.

The position of the 'in-laws' (a happy phrase which is attributed ... to her Majesty, than whom no one can be better acquainted with the article) is often not very apt to promote happiness. ["Blackwood's Magazine," 1894]

The earliest recorded use of the phrase is in brother-in-law (13c.); the law is Canon Law, which defines degrees of relationship within which marriage is prohibited.

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Online Etymology Dictionary, © 2010 Douglas Harper