The Man Trap
“Doctor, why did the Captain call you… Plum?”
McCoy twitched and turned abruptly to stare at Spock. The Vulcan stood with his head barely inclined, his hands clasped behind his back, expression dead serious.
“It’s… it’s a nickname.”
“Fascinating. And why do humans engage in this practice?”
“It shows affection. Lovers often use nicknames.”
Spock’s eyebrow came up and down. “You and the Captain…”
McCoy gaped and shook his head. “That’s what Nancy used to call me that. Jim is using it as a joke.”
“Ah,” Spock nodded as if he understood. “A joke. One of the most fascinating human behaviors.”
~
I know the episode was pretty serious after the beginning, but I'm sure at one point Kirk wouldn't be able to resist calling McCoy "Plum" and then confuse the hell out of poor Spock. "You are not purple, tasty, or round!" McCoy: "..."