Stiff drink

Why "stiff" drink?

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I've always thought it was this: put enough alcohol in a drink, and even a veteran drinker may conceivably jerk his/her head back from the glass--almost recoil--after the first taste. Conceivably the drinker's torso appears to stiffen up, although I think that this body language is sometimes affected playfully between friends as a gesture of appreciation for a well-prepared concoction.