The Hollywood icon - renowned for his prowess with the ladies - allegedly struck up a relationship with Meryl on the set of their 1987 drama Ironweed, and the pair even had sex in his trailer on set, Mark Eliot's new biography Nicholson claims.
Mark wrote: "Often during shooting, his Winnebago seemed to be balanced on four overworked Slinkys."
The pair first met in 1985 when they starred together in Heartburn, but filming ended with Meryl throwing Jack out of her hotel room and promising never to make another movie with him because of his "relentless sexual overtures", claims she denied.
They teamed up again two years later and their fling allegedly started soon after, but Mark believes Jack was still in an on/off romance with Anjelica Huston and was also sleeping with actress Veronica Cartwright at the time.
The book states: "As soon as Ironweed began, rumours exploded like wild mushrooms that something was going on between Jack and his co-star, I'll-never-work-with-him-again Meryl Streep.
"There had been talk that the two had grown unusually close, but both denied it. Jack was in a bit of a bind, as he was actively, if secretly, still seeing Cartwright.
"However, once filming began on Ironweed, everyone on set and those who heard about it, were talking not about the film's script, or direction, or scenic design, but about Jack and Meryl."
Meryl - who has been married to Don Gummer for 35 years - and Jack still remain close friends to this day.
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