Best part: this was improv.

George wasn’t supposed to have a line there, he was supposed to brandish the sword, grab her and leave. George decided to ad-lib a line because he thought it was funny and that it was in character. So Nichelle had no idea that George was about to say anything. They start the take, George leaps over there, grabs her and says his line. And Nichelle Nichols, not Lt Uhura, looks at him with That Look on Her Face, and in two iconic words lets the world know what she and Lt Uhura are about.