You may think losing $4 billion betting on a shady pharmaceutical company was the lowest point in Bill Ackman’s life. Or maybe it was the apparent failure of his shorting of multi-level marketing outfit Herbalife.

But a single tussle with former Vice President Joe Biden might have stung worse than either.

The Biden blow came at the SkyBridge Alternatives (SALT) Conference this May, but was reported by Fox Business for the first time yesterday. At the conference, where Biden also made comments about Hillary Clinton’s candidacy, Jeb Bush is said to have asked Biden why he didn’t pursue the Presidency himself in 2016. Biden offered an emotional explanation hinging on the death of his son, Beau Biden, in 2015, but cut the answer short with the comment: “I’m sorry . . . I’ve said enough.”

Ackman, his mouth apparently outrunning his brain, quipped: “Why? That’s never stopped you before.”

Biden, according to Fox sources who were present, turned to a nearby guest and asked regarding Ackman, “who is this asshole?”

He then addressed Ackman directly: “Look, I don’t know who you are, wiseass, but never disrespect the memory of my dead son!”

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One wonders whether Ackman was more wounded by the former Vice President’s harsh words, or by the fact that he didn’t know who Ackman was. But the hedge funder reportedly attempted an apology. Biden responded by telling him to “just shut the hell up.”

Investors in Ackman’s Pershing Square may wish he’d gotten that message years ago: his long-winded defense of Valeant in an October 2015 conference call ushered in years of dismal performance for the fund.

Update, June 27, 2017: In an article published Monday evening on the website of New York magazine, Ackman called the Fox Business story a “totally false press report” and professed “enormous respect for Vice President Biden.” In a statement released to New York, the SALT conference’s host, SkyBridge Capital, said that the tense exchange between the two men came during a conversation about “the current political environment and administration [, not] the untimely death of Beau Biden.” Other sources cited in the article said that Biden had called Ackman a “wiseass” but not an “asshole.”