TRUMP: Here's the backstory on my 'low-energy' takedown of Jeb Bush

It has become a signature attack line of the 2016 presidential campaign, and a label that has stuck as one that describes former Florida Gov. Jeb Bush (R): "low energy."

Donald Trump started using the "low-energy" moniker to describe Bush sometime in August, as he continued to surge in polls while Bush sunk. Today, Trump has taken over the front-running position that Bush was long presumed to hold.

In an interview with Business Insider last week, Trump said there was no backstory behind the tag - no moment where it "clicked" that he should constantly use it in interviews and on the trail. But Trump said its success as an attack line was rare - even for him.

"Yeah, that's once in a decade," he said during an interview in his midtown Manhattan office.

The interview was conducted as part of Business Insider's special report on the Trump campaign, "Trump Nation: On the trail with the GOP front-runner."

When asked if there was a deeper story behind the label, Trump added, "No, no. He just seemed to me" like a "low-energy" person.

Trump and Bush have feuded extensively during the campaign season. Among other things, Trump has repeatedly attacked Bush for his stance on immigration, and Bush has been a prime target in his series of short, quick-hitting Instagram attack videos. Bush, for his part, has gone so far as to suggest Trump is germophobic.

The "low-energy" label has riled Bush enough for him to mention it on national television.

When asked during the second Republican presidential debate in September to pick a theoretical Secret Service codename, Bush chose the battery brand "Eveready."

"It's very high-energy, Donald!" Bush said of the name.

Trump has de-escalated his feud with Bush in recent weeks, opting to go after GOP threats like Sen. Marco Rubio (R-Florida) and retired neurosurgeon Ben Carson. He told Business Insider that he doesn't think the race is "about Jeb anymore."

"I really don't. When I first ran, I hit him really hard, because I thought he was going to be the guy. You know, he's the establishment guy. So I hit him very hard," Trump said.

He added: "But he's a low-energy individual. I said that. That doesn't happen often. I've said things about Rubio that are very tough."