Spock and McCoy were on Starbase 11, overseeing the acquisition of new equipment for the Enterprise's biology labs. The Lexington was also at the starbase for resupply, and its crew was apparently on shore leave, since the corridors were thronged with Lexington personnel.
Two of the Lexington's officers walked by, observing Spock and McCoy, and once they were out of human earshot, one turned to the other and said, "I don't think I've ever seen Kirk's Shepherd without Kirk before."
The second officer laughed and said, "Guess they're not joined at the hip after all."
Puzzled by these remarks, Spock raised an eyebrow. Seeing him, McCoy asked, "What is it, Spock?"
Spock repeated the remarks for McCoy, then added, "But James Kirk is very nearly as UNsheeplike as it is possible for a human to be, nor is there anything especially pastoral in my own nature."
McCoy chuckled. "That 'shepherd' is short for 'German Shepherd,' so they're calling you Jim's dog; people've been calling you Kirk's Shepherd for about a year now." He darted a teasing look at the Vulcan. "How do you feel about that?"
Spock regarded the doctor with a teasing glance of his own. "Since the German Shepherd Dog is notable for its strength, intelligence, loyalty, obedience, diligence, protectiveness, and versatility, I admit that I find the comparison apt."
McCoy goggled at him. "You aren't gonna tell me how illogical it is to compare you to an animal?"
Spock sighed lightly. "I have become quite accustomed to the whimsy and illogic with which humans express themselves."
McCoy smiled. "I'll be damned. You LIKE being known as Jim's German Shepherd."
Spock did not dignify this with a reply ... but McCoy noticed that the tips of his ears had blushed green.