Etymology

sister-in-law

sis·ter·in·law

Today it means: The sister of one's spouse. Full definition →

Middle English suster-in-lawe
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The story

  1. Middle English · inherited fromsuster-in-lawe
  2. English todaysister-in-law

Inherited from Middle English suster-in-lawe; equivalent to sister + -in-law.

Etymology from Wiktionary, CC BY-SA 4.0.