Charlie Stevenson, 21, denies murdering Christopher Higgs, also 21, at her home in Portland Street, Boston.
Mr Higgs, from Spalding, had been in a long term relationship with Ms Stevenson and they shared a young child - but they were not always living together, a court was told.
A jury at Lincoln Crown Court heard Mr Higgs was found with a serious stab wound to the chest on Wednesday, 14 July.
He was pronounced dead at the scene in Portland Street, Boston.
Christopher Donnellan QC told the jury Mr Higgs should not have been at the property as he was on bail for an incident involving Ms Stevenson.
But the prosecution allege Ms Stevenson was happy for Mr Higgs to be there and an argument tookplace.
Mr Donnellan said: “Christopher Zac Higgs was born on 7 August 1999.
“He died of a single stab wound to his heart. That was delivered on 14 July 2021, just after midday.
“He was 21, just short of his 22 birthday.
“The stab wound with a kitchen knife was inflicted by his partner.
“The prosecution case is that the blow was deliberately inflicted, it was not an accident, it wasn’t done in self defence, or because of a mental health issue or a personality disorder which you may hear about.”
Mr Donnellan added: “There was an argument, it wasn’t uncommon for them to argue, Chris and Charlie.”
But Mr Donnellan told the jury: “She (Ms Stevenson) did not need to pick up a knife, she did not need to use a knife.”
It is alleged Ms Stevenson intended to cause Mr Higgs at least really serious harm when she delivered the blow.
Mr Donnellan said the last other person to have contact with Mr Higgs was his brother, Charlie Higgs, who had a Facetime call with his brother and Miss Stevenson from around 11.20am.
That call ended with Charlie Higgs noticing his brother and Ms Stevenson bickering - but that was not unusual, Mr Donnellan added.
The next call made was a 999 call to the ambulance service from Ms Stevenson’s phone at 12.07pm.
During the call Ms Stevenson told the operator Mr Higgs had stopped breathing properly and she was given advice on administering CPR.
Ms Stevenson remained at the scene until paramedics arrived.
The jury heard both Mr Higgs and Ms Stevenson came from a troubled background, and both had been in care, with Ms Stevenson from a very young age.
Mr Donnellan said Ms Stevenson had made reports against Mr Higgs.
But Mr Donnellan added: “Unlike Ms Stevenson he (Mr Higgs) can have no voice in this trial, that was taken away from him on 14 July.”
The prosecution allege Ms Stevenson had also used violence during her troubled past.
The trial continues on Monday.