President Obama used an undisclosed pseudonym to communicate with then Secretary of State Hillary Clinton on her private email server – shocking her top aide Huma Abedin when she learned of it.

“How is that not classified?” Abedin “exclaimed” to investigators when shown a copy of the 2012 exchange between Clinton and Obama, according to a trove of 189-pages of FBI documents dumped Friday night into Clinton’s use of the private server.

After learning that the president used email with a pseudonym — apparently to try to protect his identity — Abedin asked her interrogators if she could keep a copy of the email.

The FBI documents don’t reveal if they let Abedin take a copy home with her.

Although tens of thousands of Clinton’s emails have been made public since it was revealed she had exclusively used a private email server to conduct official work, emails with the president have been exempt from disclosure.

“This exchange strongly suggests Clinton and the president knowingly discussed classified material in an unclassified setting and hence broke the law,” Raj Shah, the research director at the Republican National Committee, told the Post.

But a rep for Clinton downplayed the documents significance.

“These materials further demonstrate why the Justice Department believed there was no basis to move forward with this case,” spokesman Brian Fallon said.

The documents also show IT workers for Platte River Networks — the external company that hosted Clinton’s server — referred in an internal work ticket to “the Hilary [sic] coverup [sic] operation” when their client asked for changes to email retention and archive/email cleanup. A worker later said the term was a joke.

The Trump campaign immediately seized on this, with senior communications advisor Jason Miller saying: “there was a concerted effort to systematically destroy potentially incriminating information.”

“It’s no wonder that at least five individuals tied to the email scandal, including Clinton’s top State Department aide and attorney Cheryl Mills, secured immunity deals from the Obama Justice Department to avoid prosecution,” Miller added, referring to news Friday of additional Clinton aides receiving protection from the FBI so they could speak to authorities.