The vilest slur in Brussels, the insult to end all insults, is “populist.” Eurocrats spit it out, rather in the manner of a teenager at a party who mistakenly takes a swig from a beer can that was being used as an ashtray. Yet, monstrous as the word is in a Eurocrat’s vocabulary, he is surprisingly vague about its meaning.

The one thing that he unequivocally understands populism to signify is “something that other people like, but I don’t.” Thus, calling for a referendum is populist. Accepting the result of a referendum is populist. Free speech for Eurosceptics is populist. Tax cuts are populist. Cutting waste is populist.

My neighbor in the European Parliament chamber when I was first elected was a hefty Belgian Christian Democrat. He used the word frequently and ferociously, applying it with particular venom to supporters of...