Relating to Xerxes son of Darius, both of them anciet Persian emperors.
Described by Herodotus, Xerxes himself was exactly as a tyrannical and nutty as an all powerful god-king ought to be and was prone to insane, disproportionate punishments and temper tantrums which included whipping the Hellespont for not obeying him.
Therefore the act of Xerxesing describes actions that are cruel, mad and over the top, usually performed in a fit of rage. Getting Xerxesed is far worse than usual punishment and is to be avoided at all costs.

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