A grumpy Robert De Niro took the stand Monday in a Manhattan court showdown with his former assistant — and in a scene worthy of Hollywood, tried to bat back claims that he was the boss from hell.

The curmudgeonly 80-year-old “Raging Bull” Oscar winner rambled and ranted while fending off accusations from longtime former right-hand woman Graham Chase Robinson, who has accused him of everything from unwanted physical contact to making lewd jokes about his Viagra prescription.

“This is all nonsense!” the legendary actor, dressed in a green polo shirt and jacket and wearing black glasses, dramatically bellowed.  

De Niro’s lawyer, Richard Schoenstein, had set the stage in his opening statements Monday, the first day of the actor’s civil trial face-off with Robinson, when he acknowledged, “This trial is going to be a little bit like a movie.’’

De Niro’s and Robinson’s now-long-running legal battle kicked off when the “Taxi Driver” actor’s production company, Canal Productions, sued her in August 2019.

The star’s camp alleged that his once-trusted employee, 41, had raided company coffers, binge-watched “Friends” and Netflix while on the clock, and even stolen millions of De Niro’s frequent flyer miles.

Robinson fired back with her own $12 million suit two months later, accusing her famous boss of 11 years of subjecting her to sexual harassment and bullying while expecting her to work 24/7 and assume the role of his “office wife.”

One of Robinson’s lawyers, Brent Hannafan, told the jury that De Niro “was a really demanding boss.”

“He would berate her. He would make her life difficult,” Hannafan said, adding that his client’s job was to do what her star boss “wanted and when he wanted.”

“He called Chase a b—h to her face repeatedly,” the lawyer added of the alleged verbal abuse.

De Niro also had her perform demeaning, physically objectionable tasks, Hannafan said.

One time, “He asked her to literally scratch her back,” the lawyer said.

When Robinson asked him why he didn’t just use the back-scratcher that was there, De Niro said, “I like the way you do it,” Hannafan said.

Robinson has said she was also on the receiving end of De Niro’s allegedly misogynistic comments — including suggesting she could get pregnant using the sperm of a married male colleague.

“He would joke with Ms. Robinson about his Viagra prescription. De Niro smirked at Ms. Robinson about his young paramour, who was around Ms. Robinson’s age,” her suit said. 

But De Niro’s lawyer fired back that Robinson “always played the victim” and painted her as a money-hungry ex-employee.

“It’s nice to work for a movie star, but it can be mundane,” Schoenstein said in trying to downplay the tasks his client may have asked of Robinson.

Robinson started working for De Niro in 2008 but claims she was “forced to resign” in April 2019 after rising to the position of vice president of production and finance.

She has alleged in her suit that De Niro assigned her “stereotypically female job duties that were inconsistent with her job title” — including putting away his boxers, washing his sheets, and setting his table.

Robinson has also alleged that De Niro’s girlfriend, Tiffany Chen, 45, falsely accused the assistant of being in love with De Niro during her time at the Hollywood legend’s company.

“Chase Robinson never had any romantic interest in Mr. De Niro. Zero, none. He was her boss and that was all,” Hannafan said, adding that Chen was “jealous” of their relationship.

De Niro testified that he initially doubted it when Chen first told him Robinson was “in love” with him — but he was now coming around to his girlfriend’s way of thinking.

“I said I could not believe that, but now, looking in hindsight, maybe she had a point,” he testified.

When pressed about Chen’s relationship with Robinson, De Niro told jurors that his former assistant had “disrespected” his girlfriend.

At one point, another Robinson lawyer, Andrew Macurdy, asked the “The Godfather II’’ actor whether he wanted his girlfriend, Chen, to be happy.

“Wouldn’t you?” responded De Niro, drawing laughs from jurors.

Robinson’s lawyers noted that the former assistant had been De Niro’s emergency contact — and that he called her around 4:30 or 5 a.m. one day in 2017 because he’d fallen down the stairs at home and needed to go to the hospital.

Macurdy asked the actor why he didn’t get help from then-wife Grace Hightower, who was in the home with him at the time.

“She was sleeping, I didn’t want to bother her,’’ the actor said.

De Niro’s lawyer had earlier claimed that Robinson had called Chen a “b—h,” “wh–e” and “psychopath” at times — and would secretly read De Niro and Chen’s private messages.

Evidence set to be introduced by the former assistant during the trial includes more than a dozen voicemails and several text messages sent by De Niro to Robinson, as well as texts about Robinson that were exchanged between De Niro and Chen, according to court records.

In one text message sent to De Niro in January 2019, Chen had written that Robinson believes “she’s your wife.”

“Her possessive manner over the house makes me very uncomfortable, down to having the cable bill and the security system password be [in] her name. It’s all very weird and she is not easing up with the weird s–t either. I do feel it’s getting worse,” Chen wrote, according to court papers.

“She thinks she’s your wife and I’m tired of her rearranging things and throwing my stuff on the floor in chaos whenever she decides she wants to be the ‘lady of the house’ it’s very bizarre and it really has to stop,” Chen texted, court papers say.

Elsewhere in her suit, Robinson charged that De Niro terrorized her with creepy conduct over the years — including moments when he “urinated during telephone calls” with her and would greet her wearing only his “pajamas or a bathrobe.”

Robinson has also alleged that she was on the receiving end of the A-lister’s expletive-filled tirades, including one 51-second voicemail recording that she once received when she didn’t pick up her phone.

“You f–king don’t answer my calls. How dare you? You’re about to be fired. You’re f–king history,” De Niro says in an undated recording of the message included in the suit. “This is bulls–t. How dare you f–king disrespect me? You gotta be f–king kidding me, you spoiled brat! F–k you!”

De Niro, for his part, has claimed Robinson abruptly quit her $300,000-a-year job when “suspicions arose” regarding her “honesty, integrity, work ethic and motivation.”

After being asked a question Monday about Robinson requesting vacation time at one point, De Niro interrupted, “Can I ask a question?”

Then he said, “What’s all this about a vacation?”

Judge Lewis Liman warned him, smiling: “I’m the one who asks the questions.”

A review of the books after Robinson left the job found that she used the company’s American Express card to spend tens of thousands of dollars on trips, iPhones, Ubers, a Louis Vuitton handbag, dog-sitters, and pricey dinners, court papers allege.

De Niro’s suit also claims Robinson “loafed during work hours, binge-watching astounding hours of TV shows on Netflix.”

After Robinson abruptly resigned, De Niro’s lawyer told jurors, the former assistant demanded a two-year severance package, health care, and even a press release announcing her departure from the company.

Robinson claimed her relationship with her now-ex-boss quickly started to sour as their negotiations continued.

She fired off an email to De Niro and his lawyer, Tom Harvey, in July 2019 threatening to obtain her own legal representation if they didn’t respond by a certain date, according to court filings.

De Niro raged in a text to his girlfriend, “Can you believe Chase. As I said to Tom: who the f–k does she think she is?!?!”

Chen replied, “She thought she was your wife. I saw it from the beginning. I told you,” to which De Niro responded, “The balls. the nerve. the chutzpah. the sense of entitlement, how dare her!”

The trial is expected to last two weeks.