Heard the one about the jester who became a kingmaker?

No joke: Last week’s election results in Italy gave the center-left Five Star Movement (M5S) roughly a quarter of the vote — and a decisive hand in the forming of a new national government.

That’s serious business for M5S leader — and popular comedian — Beppe Grillo.

He has called former Prime Minister Silvio Berlusconi, a conservative of sorts (at least by Italian standards), “the psycho dwarf.” He has called Pier Luigi Bersani, head of the center-left Democratic Party, “a dead man talking.”

And the 64-year-old entertainer calls for ending the rampant corruption of Italy’s political class and starting on a path away from unsustainable government debt.

Though certainly no conservative, Mr. Grillo did deliver an enduring line on government’s financial limitations a quarter century ago. As he told the gag, when Italian Socialist leader Bettino Craxi visited China, a puzzled member of his party asked him: “If the Chinese are all socialists, whom do they steal from?”

Today, with Italy’s ratio of public debt to gross domestic product eclipsed by only Greece among euro-zone nations, Mr. Grillo advocates returning to the lira: “Right now we are being crushed, not by the euro, but by our debt.”

Most elected officials in Italy — and most economists there and elsewhere — rightly scoff at the naive notion that the nation could somehow strengthen its balance-sheet hand by dropping the euro.

Yet that’s not stopping some major Italian political players from currying Mr. Grillo’s favor after last week’s election stunner.

Gaining his approval, however, apparently won’t be easy.

He told The New York Times last weekend that for him to help form a new national government “would be like Napoleon making a deal with Wellington.”

Of course, even without such a deal, Napoleon met his Waterloo.

Beyond Italy’s current political — and economic — turmoil, though, lies this troubling reality:

Any nation that spends far enough beyond its means for long enough will eventually meet a fiscal Waterloo of its own.

And that’s no joke, either.