First no offense to anyone, esp. people from Mongolia.

Mongolian in Chinese can either mean
a. people from Mongolia
b. not smart, retarded

If someone is called a "mongolian doctor" it means that his medical practice is sub-par at best.

I believe in the case of mongolian beef, it simply means untasty beef - in a twisted sense of chinese humor. Although as a Chinese, I find mongolian beef actually not *tasty*.

Again, not to offend anyone, just my 0.02.