Jack Nicholson learned his sister was actually his mum after both of them died

The 'most f***ed thing [he'd] ever heard'

Upon learning the truth, the Shining actor deemed it the 'most f***ed thing [he'd] ever heard'.

Jack Nicholson was born in 1937 in Neptune, New Jersey. His mother, June, was only 18 when she became pregnant, and the father, Don, was already married to another woman. To avoid scandal, June’s mother, Ethel May, who was Jack’s grandmother, pretended to be his mother instead. June was introduced to him as his much older sister, while her sister Lorraine was presented as another sibling.

When Jack was 17, he moved to Los Angeles, where June had also relocated in hopes of pursuing an entertainment career.

He told Rolling Stone back in 1986: "Since my only relative in the world was June, who was out here, I came out to look around."

Jack began working in the animated cartoon department at MGM Studios as an office boy. However, his charm and knack for acting didn't go unnoticed, leading him to pursue acting.

However, Jack was in for one of the most shocking plot twists off-screen in 1974, just when his critically acclaimed film Chinatown was about to hit cinemas.

It was an interview for Time magazine that uncovered the truth about Jack's family, that Ethel May was actually Jack's grandmother, and that his 'sister' June was in fact his biological mother.

To add to the shocking truth, they further claimed that Jack's father was still alive and living in Ocean Grove, New Jersey.

After learning the truth, Jack immediately called Lorraine's husband Shorty, demanding answers to this elaborate lie.

At first, Shorty denied the claims, however, he later revealed the claims were true, but he couldn't confirm if the man the researchers at Time Magazine had found was really Jack's biological father.

By the time the truth came out, June and Ethel May had already passed away, meaning the actor was never able to confront them.

For many, this revelation would be devastating, however, Jack insisted it wasn't something that deeply 'traumatised' him.

"I was very impressed by their ability to keep the secret, if nothing else," he later said in The Rolling Stone interview.

"It’s done great things for me."