An 80-year-old Queens cougar — whose hulking much-younger boyfriend was arrested over the weekend for beating her up — was found dead in her apartment Thursday, law-enforcement sources said.

Janina Olszweka was lifeless and had visible trauma to her head and body when her son found her, sources said.

The victim’s boyfriend, Marek Piotrkowicz, had been arrested for domestic assault against her five days earlier, allegedly leaving her with bruises on her face and minor cuts.

Piotrkowicz, 56, was charged with assault and endangerment in the incident, then released by Queens Criminal Judge John Zoll on his own recognizance hours later, according to the Queens District Attorney’s Office. Prosecutors had asked for $5,000 bail.

The 6-foot-6 boyfriend has two priors, both sealed, sources said.

Police were interviewing him Thursday, they said. Police also have talked to the woman’s son.

Sources said Olszweka was an alcoholic who also routinely fought with her son, Irek.

Irek told cops he found his mom sleeping on her floor last night and called 911 the next morning when he found her in the same spot.

Neighbors say Olszweka, a former maid at a hotel, lived in the Ridgewood apartment for at least 20 years with her son.

“Sometimes I would hear them screaming when I took out the garbage,” said building super Dzemail Praprankiu.

Neighbor David Perez said authorities often responded in person during the mother-son spats.

“Cops would be called here several times, and they would talk to him in the hallway,” he said. “She could barely walk.’’

Additional reporting by Larry Celona and Linda Massarella