But we say that as the meaning of "Not at all." in spoken Korean, not as the meaning of "You're welcome." When we see foreign movies with Korean subtitle, it gets taken as the meaning of "You're welcome." You can see the origin from 千不當萬不當(千不当万不当 in simplified Chinese characters) 千 = 1000, 不 = not, 當 = proper/appropriate. This is how the Korean dictionary says about it.