This redefined “fit for a king.”

Just in case there weren’t enough royal revelations of late, the disgraced Prince Harry revealed a particularly juicy bombshell regarding his father King Charles III’s alleged daily exercise regimen. The humiliating tidbit surfaced via the Duke of Sussex’s tell-all memoir “Spare,” which dropped Tuesday.

According to the 38-year-old, Charles would frequently do handstands in just his skivvies to help alleviate “constant neck and back pain” due to “old polo injuries.”

“Prescribed by his physio, these exercises were the only effective remedy for the constant pain in Pa’s neck and back,” Harry described. “He performed them daily, in a pair of boxers, propped against a door or hanging from a bar like a skilled acrobat.”

In fact, the duke dreaded knocking on the doors of Balmoral’s 50 rooms out of fear of discovering his dad doing underwear handstands on the other side.

“You might blunder in as he was doing his headstands,” said Harry, describing a typical “brief” encounter. “If you set one little finger on the knob you would hear him begging from the other side, ‘No! No! Don’t open! Please God don’t open!'”

It’s yet unclear if His Highness, now 74, still suffers from chronic pain or if nearly naked headstands remain part of his exercise regimen.

King Charles’ morning routine is far from the only revelation to emerge courtesy of “Spare.”

Other leaked excerpts describe Harry losing his virginity to an equestrian cougar, suffering from a “frostbitten penis” at his brother’s wedding and amassing a massive kill count in Afghanistan.

“Spare” is currently available in bookstores as well as on Amazon in both hardcover and Kindle.

Readers can also hear the royal family secrets straight from the horse’s mouth by buying the Prince Harry-narrated audiobook on Audible.

It costs just $5.95 a month for the first four months, for a limited time only.