OHMYGOSSIP — Jack Nicholson used to sleep with a hammer under his pillow. At the time, the actor was involved in a bizarre Hollywood love triangle, was under the influence of drugs, and worried he’d be killed.

The 76-year-old’s dirt is dished in the blockbuster tell-all Nicholson: A Biography, slated to be published Oct. 29.

Jack feared for his life while involved with The Mamas and The Papas singer Michelle Phillips.

Nicholson was introduced to Phil­lips in 1970 after the sexy blonde singer had just divorced Dennis Hopper, Nicholson’s Easy Rider costar.

Hopper and Phil­lips had managed to stay married for only eight days before a physical brawl destroyed their relationship.

When Jack and Michele got together, was convinced Hopper would come after him seeking bloody re­venge. Jack, 36 at the time, was so paranoid, he slept with “a hammer under his pillow,” according to best-selling author Marc Eliot.

When the two actors finally met to talk about Jack’s relationship with Mi­chelle, Hopper just laughed it off.

Eliot says that Jack struggled with a gay secret. While under the influence of LSD, the actor experienced “castration fantasies” along with “homoerotic fear fantasies”.