Republican presidential nominee Donald Trump. (Jonathan Ernst/Reuters)

WHERE DID Donald Trump get the harebrained idea that he will get Mexico to pay for a new border wall? Or his boast that he will make Arab states pay for refugee safe zones in Syria? Perhaps from the success he has had sleazily diverting money other people had donated to charity into paying off his business obligations.

"It's called OPM. I do that all the time in business. It's called other people's money," Mr. Trump saidTuesday. "There's nothing like doing things with other people's money."

One of the things he did with other people's money, according to revelations painstakingly uncovered by The Post's David A. Fahrenthold, was fund his namesake charity, from which donations appeared to come from him. Mr. Trump then used some of that funding to pay off business liabilities. In one instance, he settled a dispute with the town of Palm Beach, Fla., over $120,000 in unpaid fines by promising to donate $100,000 to a veterans group. Instead of donating personal funds, he transferred some money — OPM — from his charitable foundation. Similarly, the GOP nominee used other people's charitable giving to settle a dispute over a hole-in-one tournament held at the Trump National Golf Club in New York. At the time, his foundation was mostly funded by a big donation from pro-wrestling magnates Vince and Linda McMahon.

These revelations come on top of findings that Mr. Trump and his wife used foundation money to win items at charitable auctions, such as a helmetsigned by quarterback Tim Tebow and not one but two paintings of Mr. Trump.

It is likely that this sort of behavior is illegal, because charities are not supposed to engage in "self-dealing" — activities that directly benefit the bottom lines of those running them. But even if Mr. Trump's approach to "philanthropy" is not technically illegal, it is still slimy and shameless. Sometimes charities associated with wealthy families or businesses tiptoe near ethical lines — but charity experts say Mr. Trump's activity appears to be brazen.

"Brazen" also describes the Trump campaign'sresponse to The Post's reporting. As the revelations have mounted, Trump spokesmen have accused The Post of "badgering" them and said that Mr. Fahrenthold is "biased" and "a little obsessed." Though the campaign insisted that The Post's reporting "is peppered with inaccuracies and omissions," it pointed to none and has offered no evidence telling a different story.

In fact, Mr. Trump has a secret trove of documents that could help to clarify how much he has really given to charity in recent years, what his business dealings look like and potential conflicts of interest should he be elected president. These documents are his tax returns, which he still refuses to reveal, bucking decades of precedent. Mr. Trump is the least transparent major-party presidential nominee in recent memory — and the one who Americans have the most reason to fear is hiding something.