Hillary Clinton was so oblivious to her duty to guard the nation’s secrets that she didn’t even realize that “(C)” in an e-mail header means “confidential.” And that’s just one of the revelations in the FBI report on its investigation into her abuses.

In addition to the thousands of Clinton e-mails the State Department has found to hold classified data, the FBI ID’d 81 classifed e-mail chains — eight of them “top secret.”

Clinton used at least 13 mobile devices at State. Some were properly destroyed afterward — but others just went missing.

She didn’t even know how classification worked: “Clinton could not give an example of how classification of a document was determined.” And, in one example, she didn’t even realize that “deliberation over a future drone strike” — a highly classified issue — was “cause for concern regarding classification.”

After news broke of her private setup, she tried to completely wipe all her records. In the end, she failed to hand over at least 17,448 work e-mails.

Though she denied it again to the FBI, her goal throughout was plainly to evade the Freedom of Information laws: Whenever anyone else — from Colin Powell to techies at State — noted that a course of action would mean leaving records that might become public, she chose a different path.

In her FBI interview, she “couldn’t recall” any State training or warnings on retaining records, handling classified info or e-mail policies of any kind.

But she was the boss: It was her job to know the rules for protecting the nation’s secrets and leaving a complete record of her work — or at least to hire aides who’d get it done. But she hired enablers who helped keep her secrets without guarding America’s.

In total, the report shows behavior far worse than “extremely careless,” as FBI chief Jim Comey termed it as he recommended against legal charges. Central to the case for mercy was his claim she didn’t “intend” to break the law. Yet his own agents’ report shows that, at the minimum, she just didn’t give a damn what the law required.

“Criminally reckless” is more like it.