One of the mysteries of the English language finally explained.
1A woman or girl who shares at least one birth parent with another person; a sister by birth.
2A woman or girl who has pledged to treat another with loyalty or trust, typically in a ceremony involving the mingling of blood; a trusted friend or companion.
Mid 19th century; earliest use found in Church Review. From blood + sister.
blood sister/ˈblʌd sɪstə/
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