He didn’t actually say the word “default”, but what he is describing here is a default (“not repaying loans in full”):

One day after assuring Americans he is not running for president “to make things unstable for the country,” the presumptive Republican nominee,Donald J. Trump, said in a television interview Thursday that he might seek to reduce the national debt by persuading creditors to accept something less than full payment.

Asked whether the United States needed to pay its debts in full, or whether he could negotiate a partial repayment, Mr. Trump told the cable network CNBC, “I would borrow, knowing that if the economy crashed, you could make a deal.”

He added, “And if the economy was good, it was good. So, therefore, you can’t lose.”

Such remarks by a major presidential candidate have no modern precedent. The United States government is able to borrow money at very low interest rates because Treasury securities are regarded as a safe investment, and any cracks in investor confidence have a long history of costing American taxpayers a lot of money.

Experts also described Mr. Trump’s vaguely sketched proposal as fanciful, saying there was no reason to think America’s creditors would accept anything less than 100 cents on the dollar, regardless of Mr. Trump’s deal-making prowess.

“No one on the other side would pick up the phone if the secretary of the U.S. Treasury tried to make that call,” said Lou Crandall, chief economist at Wrightson ICAP. “Why should they? They have a contract” requiring payment in full.

This is unprecedented, as is many of things Trump says and does.  This would lower America’s credit rating and our future ability to borrow.

This is precisely why business acumen does not translate into political acumen.  Defaulting on a loan is a business decision.  So is bankruptcy.  It can be done and the company can bounce back. But you can’t think that way with a country.

Oh my God. I don’t understand how even fiscal conservatives can swallow this.

The Business Insider headline reads: “Donald Trump is floating an insane idea that would tank the American economy” —

“I’ve borrowed knowing that you can pay back with discounts,” he said. “I would borrow knowing that if the economy crashed, you could make a deal.”

Some corporate finance deals really do work like this: You issue risky debt, and the lenders know you might not be able to pay them back in full if something really bad happens. But that kind of debt bears a high interest rate, because the lenders know you might not be able to pay them back in full if something really bad happens.

US Treasury bonds have very low interest rates because investors are extremely confident they will be paid in full, even in poor economic conditions. Trump — by openly saying that he would keep partial payment on the table as an option — could spark a crisis in the Treasury markets if he became president. Investors would cease to see Treasurys as a safe asset, and they would demand higher interest rates in exchange for risk.

This, of course, is a terrible idea, and a good reason for Republicans to hesitate in coalescing around Trump.

Maybe Trump should read Section 4 of the 14th Amendment that says this:

The validity of the public debt of the United States, authorized by law, including debts incurred for payment of pensions and bounties for services in suppressing insurrection or rebellion, shall not be questioned. But neither the United States nor any state shall assume or pay any debt or obligation incurred in aid of insurrection or rebellion against the United States, or any claim for the loss or emancipation of any slave; but all such debts, obligations and claims shall be held illegal and void.

UPDATE: 

Pressed to elaborate on his remarks, Mr. Trump did appear to step back. He said that he was not suggesting a default, but instead that the government could seek to repurchase debt for less than the face value of the securities. The government, in other words, would seek to repay less money than it borrowed.

Jesus.  Is this what a Trump presidency will be like?  Him doing things and then walking them back?