Flipping through The Wrath of Khan Photostory today, I was reminded of Kirk's fondness for what are probably antiquated phrases by the 23rd century. They're old by modern standards. In The Wrath of Khan, they're often directed at Saavik. She must be baffled at expressions like "caught with my britches down" and "sauce for the goose".
I think it's likely that other bridge members are just as confused. I love to think about Kirk rattling these sayings off and everyone silently wondering what the hell he means.
On the other hand, expressions tend to outlive their literal representations. Maybe everyone knows what it means to be caught with your britches down without actually knowing what britches are. Maybe people think it's "caught with your bridges down".
In any case, we know Kirk loves Earth history. He's one of those people who are easy to buy presents for because they have definite interests. His two best friends both buy him antiques. If he wasn't a Starship Captain, I imagine it would be a full-time fandom for him. As is, I think he seeks out interesting tidbits like old slang. If he's especially delighted by a phrase, he starts incorporating it into his speech.
