A hangover, usually involving whiskey; may be mild or severe
"I’ve still got a touch of the Irish flu."

"The Irish flu?"

"Also known as a hangover."

(from "A Nose for Murder" by Lee Charles Kelley
by Lee Charles Kelley April 06, 2005
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The ailment one suffers on March 18th after properlyly celebrating St. Patrick's Day. Anyone of Irish descent may also use the term to denote a hangover on any day of the year.
Seamus: Ugh, I was out drinking Guinness until the wee hours. I think I'll call in with the Irish Flu today.
by robbie vienna March 17, 2009
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