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It's in the first episode. George was supposed to help Burke with a surgery, but he chickened out. The other interns started to call him 007, which means Liscence To Kill. Which is refrenced from James Bond.
And when George wrote 007 on Meredith's hand, she realized that it was him.
I'm afraid that some people still don't know what the concept of licence to kill, and the code of 007 means. In a nutshell: you are allowed by a government to kill somebody without proofs, but more it is used for foreign actions. Here is better explained : http://www.licencetokill.info/the-concept.html .