sister-in-law

[sis-ter-in-law]

noun

plural

sisters-in-law 

1. the sister of one's spouse.

2. the wife of one's brother or sister.

3. the wife of one's spouse's sister or brother.

sister-in-law

noun

1. the sister of one's husband or wife

2. the wife of one's brother

Word History and Origins

Origin of sister-in-law¹

First recorded in 1400–50; late Middle English syster in lawe; sister, in, law ¹; from Middle English in-lawe “in law,” i.e., “a person within the regulation and protection of the law,” based on the prohibition by Roman civil law and, later, Christian canon law, of marriages within four degrees of consanguinity, i.e., up to and including first cousins

Related Words

• aunt 
• cousin 
• father 
• folk 
• mother 
• niece 
• sibling 
• uncle