When a sickness is passed around after attending a fraternity event.
Sarah: "Why is everyone so ill after this weekend?"

Anna: "oh you haven't heard? The frat flu is going around from ligma Chis event this weekend."
by EFC1222 April 27, 2021
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The random sickness that a geed gets when he spends too much time talking to his exclusive situationship that “goes to another school” instead of hanging with the boys.
Dude our boy Mike is coming down with the frat flu, he never hangs with us anymore.
by Bigdog cowboy September 10, 2023
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