A Piece of the Action - S2-E17
Spock:Young man, this is likely to be quite hazardous. If I were you...
Kirk: Hold on, Spock. Out of the mouths of babes...
Kid: Who ya callin' a babe?
Kirk: I'm callin' you a babe.
Kid: You callin' me a babe?
Kirk: Yeah, I'm callin'... [Kid brandishes a knife, Kirk continues.] I'm callin' you a babe, but there's nothing personal. Sit down.
Answer: Kirk was getting his physical and Dr. McCoy probably turned off communications, because if he hadn't, Kirk would have left and headed straight for the bridge, leaving McCoy irritated.