A 136kg woman from Eerie, Pennsylvania in the USA, has plead guilty to murdering her 54kg boyfriend in March by sitting on him.

Windi Thomas (44), who has yet to be sentenced, now faces jail time for the third-degree murder of Keeno Butler (44), Metro reports. In the US, a third-degree murder carries a recommended sentence of 18 to 36 years.

This past Monday the Erie County court heard how Thomas had stabbed Butler in the hand and beat him with a table leg before sitting on his chest and smothering him, Erie News reports.

Defence attorney Mark Del Duca argued that the murder was unintentional and that Thomas collapsed on top of her boyfriend because she was drunk.

"[The] actual cause of death is she collapsed on him and he basically died of suffocation," Del Duca said.

Sandra Jean Butler, the victim's sister, is unhappy with the ruling.

"I feel like she should have 40 [years] or more. She deserved life," she said.

Sandra went on to say that the couple's two-year relationship had been turbulent, and revealed how she'd once begged Thomas not to kill her brother.

Butler leaves behind his mother, daughter, two grandchildren and nine siblings, according to his obituary published on The Meadville Tribune.

Sources: Erie News, Metro, Daily Mail