When you try to say kill yourself in the chat, but the chat filter won't let you so you have to say Krill Yourself.
Guy: *Snipes you*
You: KRILL YOURSELF!
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This phrase is essentially saying "kill yourself" except it replaces the first word with a small, shrimp-like crustacean.
"You should krill yourself, NOW!"
"How could you say such a thing?"