007 is a nickname given to surgeons who tend to kill their patients, originating from James Bond's code number, which means "License to kill."
In A Hard Day's Night, George was asked to assist in an appendectomy and almost killed the patient, earning himself the nickname 007.
In Who's Zoomin' Who?, Izzie convinced Cristina to perform an unauthorized autopsy on the patient who died on their watch to save her from the shame of being the "new 007".
In Now or Never, George was hit by a bus and left unrecognizable. To let Meredith know who he was, he wrote "007" on her hand repeatedly.