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A Scottish woman who doesn't care about rules and will do anything to get what she wants, even if it means talking trash about your mom.
My cousin is a gallus besom. She stole my lunch money and said my mom was a bad cook.
The teacher called her a gallus besom because she cheated on the test and laughed in my face.
He asked her out, and she said no and called him a baby. Classic gallus besom.