Toward the end of Monday night’s presidential debate, Donald Trump released a torrent of incoherent words that seemed to revolve in some fashion around … Rosie O’Donnell?

You know, Hillary is hitting me with tremendous commercials, some of it’s entertainment, some of it is said somebody's been very vicious to me, Rosie O’Donnell, I said tough things to her and I think everybody would agree that she deserves it and nobody feels sorry for her, but you want to know the truth, I was going to say something extremely rough to Hillary, to her family, and I said to myself, I can't do it.

Parsing the sentence, Trump appears to be referring to one of Clinton’s most recent attack ads, in which a series of young girls look sadly at themselves in a mirror as various sexist Trump sound bites play: “She’s a slob,” “Does she have a good body? No,” and so on.

Trump seems to be trying to say that in all of those sound bites, he was talking about Rosie O’Donnell. No one, he’s saying, could fault him for saying that O’Donnell is a fat, ugly slob: After all, she was mean to him (he implies), and so she deserves it.

Leaving aside the fact that only one of the quotes in Clinton’s ad was referring to O’Donnell — others were about Kim Kardashian and Nicollette Sheridan, as well as women in general — it’s a bizarre moment. After Clinton discussed foreign policy and energy and whether America has a problem with systemic racism, Trump stayed true to his reality TV roots and ended a presidential debate by bringing up his old feud with a comedian.

But it wasn’t the first time Trump’s mentioned O’Donnell in a debate. During the Republican primaries last summer, moderator Megyn Kelly noted to Trump, “You call women you don’t like ‘fat pigs,’ ‘dogs,’ ‘slobs,’ and ‘disgusting animals.’”

"Only Rosie O'Donnell," replied Trump.

Given that he keeps bringing it up, the question must be addressed: Just what did O’Donnell do to get so thoroughly under Trump’s skin?

O’Donnell said true things about Trump once, and he’s never forgiven her

The public feud between the two dates back to 2006, when O’Donnell ripped into Trump on The View, calling him a “snake-oil salesman” and saying, “I don’t enjoy him.” In the process, she managed to hit almost every single talking point that’s been proven to infuriate Trump.

To put O’Donnell’s points in context: Trump had just held a press conference to announce that he would not be stripping Miss USA Tara Conner of her title after she was caught using drugs and drinking while underage. “I’ve always been a believer in second chances,” Trump said.

O’Donnell was less than amused by Trump’s show of benevolent mercy. “There he is, hair looping,” she said, pushing her hair over her head to indicate the signature Trump-over as the audience applauded gleefully. “He’s the moral authority? Left the first wife, had an affair, left the second wife, had an affair, had kids both times, but he’s the moral compass for 20-year-olds in America. Donald, sit and spin, my friend!”

O’Donnell’s co-hosts tried to come to Trump’s defense, noting, “Say what you want to say about him, but he’s a businessman.”

But O’Donnell was having none of it: “He’s been bankrupt so many times!” she cried. “The people that he owed money to got shorted out, but he got to try again and again.” She also pointed out that Trump has made a great deal of use of both the money his father lent him and the money he inherited after his father’s death. “This is not a self-made man,” she said.

So let’s review: She mocked his hair. She made fun of his attempts at gravitas. She said that he was a bad businessman, that he had gone bankrupt, and that he was not self-made.

All O’Donnell had to do, really, was call Trump a short-fingered vulgarian and she would have hit every square on the “how to piss off Donald Trump” bingo card.

Predictably, Trump was outraged. And predictably, he responded by calling her a fat loser and threatening to sue. He gave an exclusive to People magazine:

"You can't make false statements. Rosie will rue the words she said," referencing her claims that he went bankrupt. "I'll most likely sue her for making those false statements -- and it'll be fun. Rosie's a loser. A real loser. I look forward to taking lots of money from my nice fat little Rosie.”

Incidentally, there is no evidence that Trump ever went forward with his plans to sue O’Donnell, probably because she was telling the truth: He has gone bankrupt multiple times.

Trump has spent the past decade directing seething rage at O’Donnell

Instead of suing, Trump has spent the past 10 years steadily peppering O’Donnell with a stream of insults: a tweet calling her a loser here, a video calling her a disaster there. He’ll even go out of his way to loop her into the conversation for a chance to insult her, like the time he turned his Cher roast into a sick Rosie O’Donnell burn for no apparent reason:

.@Cher attacked @MittRomney. She is an average talent who is out of touch with reality. Like @Rosie O'Donnell, a total loser! — Donald J. Trump (@realDonaldTrump) May 10, 2012

He’ll occasionally throw her a bone of civility, as he did in 2012 when O’Donnell had a heart attack:

.@Rosie, get better fast. I'm starting to miss you! — Donald J. Trump (@realDonaldTrump) August 21, 2012

And every now and then he even seemed confused as to whether he liked her or hated her:

Rosie is crude, rude, obnoxious and dumb - other than that I like her very much! — Donald J. Trump (@realDonaldTrump) July 11, 2014

But for the most part, Trump’s kept his Rosie O’Donnell narrative succinct and consistent over the past decade: She is a loser and she’s fat and she was mean to him and he hates her.

For her part, O’Donnell has been vocal about how painful she’s found the whole experience. “Probably the Trump stuff was the most bullying I ever experienced in my life, including as a child,” she’s said. “It was national, and it was sanctioned societally. Whether I deserved it is up to your own interpretation.”

“You started it,” Trump replied.

This decade-long grudge shows how unfit Trump is to be president

The O’Donnell reference was just one weird blip in a frankly bizarre debate, but it shouldn’t be ignored. It points to a deeper issue: Trump is incapable of dropping a grudge. A comedian said true things about him once on a daytime talk show, and Trump is still so furious about it that he can’t stop talking about it, even 10 years later during a presidential debate.

On a night when Trump tried to claim that he had a presidential temperament, the O’Donnell feud once again proved otherwise.

Men shouldn't fear women. Trump should.