To say that people are “full of piss and vinegar” is to say that they are brimming with energy. Although many speakers assume the phrase must have a negative connotation, this expression is more often used as a compliment, “vinegar” being an old slang term for enthusiastic energy.
Some try to make this expression more polite by substituting “pith” for “piss,” but this change robs it of the imagery of acrid, energetically boiling fluids and conjures up instead a sodden, vinegar-soaked mass of pith. Many people who use the “polite” version are unaware of the original.
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