Derived from American (Hollywood) films and TV shows that contain characters, who make this kind of promise.
1st Character: "Oh, no she's gonna die!"
2nd Characer: "No she's not. She's gonna live. I promise."
"I'll be there - I promise." Howard Langston (Arnold Schwarzenegger - Jingle All The Way), In response to his son, Jamie, asking him to make it to his Karate Class.