Examples
““Cousin Malrue will have kittens when she hears that!””
Lists
These user-created lists contain the word ‘have kittens’.
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The Enigma of British Slang
At least to me it is.
completely hatstand, squiffy, kerfuffle, have kittens, bluebottle
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Feline words -- meow!
felis domesticus, felis catus, felidae, bastet, sphinx, pussy, puss, pussycat, felinophile, felicide, felinity, philofelinish and 19 more...
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The Best of British Slang
From the Dictionary of British Slang: Knickers in a Twist, by Jonathan Bernstein
lager lout, chiseler, play truant, wangle, belter, corker, butter up, diamond geezer, full of beans, jammy, collywobbles, a good seeing to and 34 more...
Tweets
Looking for tweets for have kittens.

jinglebelljosie to panic, british slang Jun 13, 2009
asativum A friend of mine has kittens. They're very cute and cuddly. Not angry or disapproving in the least. Jun 28, 2008
sassmews to get very angry; to react with disapproval
Jun 28, 2008