What do enemies mean when they shout 'plough the lily'?

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In the prologue I hear a good many enemy soldiers shout this refrain as they charge at me. I presume when they say 'plough' they don't mean it in the agrarian sense, but I think I'm missing some cultural context here because I don't quite understand how Geralt is 'the lily'.
What exactly do my enemies mean when they scream 'plough the lily'?
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