Extremely angry. The source of this expression is a bit puzzling, since hens, while not waterfowl, are not distressed by wetness. Conjecture has it that it comes from a farmer’s tossing a bucket of water at hens and thereby causing a flapping uproar. The term appears in Money in the Bank (1942) by that noted purveyor of clichés, P. G. Wodehouse. A more logical variant is mad as a hornet. H. L. Mencken referred to it as a familiar simile in The American Language (1919), and it continues to be used.