Usher and Tameka have been going through a bitter legal fight over custody of their two young children -- a fight that came to a head in a Georgia courtroom this past week.

Tameka took the stand and, as you can see in this video, was questioned by Usher's attorney about her verbal and abusive threat towards the singer -- where she allegedly said "I will kill you both," referring to Usher and his current girlfriend.

Foster went back and forth with her answer before coughing up a half-admission, saying she didn't threaten to "kill" him ... but admitting, "I did probably, out of anger, say 'I will f**k you up' and it's not acceptable."