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A way to tell someone they're a backstabbing coward who would sell their own grandma for a discount. It comes from the movie 300, where a guy named Ephialtes flipped sides and got stabbed in the back for it.
'I knew you'd trade in 300' when my best friend leaked my secrets to my crush.
'I knew you'd trade in 300' after my mom sided with my brother over me.
'I knew you'd trade in 300' when my dog ran away with my dad's shoes.