Disgraced US megastar Bill Cosby paid $3.38 million to a former university employee who accused him of sexual assault, a court heard Monday in a dramatic start to the first such celebrity trial of the #MeToo era.

The now frail and isolated 80-year-old Cosby could spend the rest of his life behind bars if convicted in his retrial of drugging and molesting Andrea Constand, 44, at his Philadelphia home in 2004.

The case has besmirched the legacy of an actor once adored by millions as "America's Dad" for his role as lovable father and obstetrician Cliff Huxtable on hit 1984-92 television series The Cosby Show.

The pioneering black entertainer's first trial ended in a hung jury in June last year, with a sequestered panel hopelessly deadlocked after six days of testimony and 52 hours of deliberations.

But before that, the case was initially settled by a civil suit in 2006, in which prosecutor Kevin Steele revealed the actor paid Constand $3.38 million as part of a previously undisclosed settlement.

In long, at times rambling remarks kicking off perhaps the most high-profile case of his career, Steele rehashed the details of the "he-said, she-said" case, saying that it was about broken trust.

He quoted liberally from Cosby's own deposition, given in 2005, in which the actor said he gave Constand an over-the-counter antihistamine to relieve stress and that they had consensual relations.

In that same deposition, Cosby admitted obtaining sedatives with the aim of having sex.

Steele sought to neuter any attempt by the defense to portray Constand as a scheming money-grabber by revealing the sum paid and saying his office had approached her -- not the other way -- once new evidence came to light.

"We're very confident that you will convict the defendant on three counts of aggravated, indecent assault for what he did to Andrea Constand on that night in January 2004," Steele told jurors.

"She was incapable of consenting to anything," he added.