Blac Chyna Threatened to Kill Kylie Jenner According to Kris Jenner

Blac Chyna threatened to kill Kylie Jenner ... so claims Kris Jenner in shocking testimony.

Kris took the stand Thursday in Chyna's trial, claiming both Kylie and Tyga once told her Chyna threatened to off the youngest Jenner.

Kris hurled the allegation right out of the box when she began testifying. When Chyna's lawyer initially asked the question -- whether Chyna ever threatened to kill Kylie -- Kris said she did not recall. The lawyer then pointed to Kris' deposition to refresh her memory, and during that depo Kris indeed did make the allegation.

When further pressed, Kris told the lawyer she would have to ask Kylie and Tyga because they're the ones with first-hand information. She did back peddle a bit, saying, "I probably thought it was just some drama, which I'm used to."

And this is crazy ... Kris told the jury after hearing about the alleged threat she still supported Rob's engagement to Blac Chyna. She said, "I love second chances, and I wanted them to win. I wanted my son to be happy." She added she had to rely on the assumption Chyna wasn't serious.