an alcoholic drink, perhaps because the word "tight" is slang for drunk"
► uses
Uses:
Daisy: Look, there's a pub up ahead. If I'm going to be a witness to one of our Rose's big scenes then I want a stiffener.
Hyacinth: We can't possibly find time...
Ms. Fortescue: Of course we can! The girls need a stiffener, and after hearing that slice of poor Rose's life, so do I!
BBC. Keeping Up Appearances: Driving Ms. Fortescue
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in less time than he would take to bring down a snipe, he was again beside her with a large mugful of hot negus, into which he had put, by way of stiffener, a copious dash of mountain dew.
Punch, or the London Charivari (August 14, 1841)
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The colonial-era day didn't begin until after a dram of bitters or stiffener of beer. By the time the Revolutionary War began, the adults of the thirteen colonies drank an impressive amount of alcohol—the equivalent of several shots every day.