Having the character or nature of Cain; marked by violence, betrayal, or wickedness, especially against one's own family.
From Cain plus the suffix '-ish' meaning 'somewhat like' or 'characteristic of'. This informal adjective construction became common in English to describe behavior reminiscent of biblical villainy, particularly fratricide.
When Shakespeare wanted to describe a character as particularly evil or treacherous toward family, he might call them 'Cainish'—the word captures that idea of betraying someone close to you, which feels uniquely despicable.
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