Jim buys McCoy a shirt that says "save a horse, ride a cowboy" as a joke. Spock takes it very seriously
Spock didn’t understand Bones’ shirt, nor the slightly embarrassed grin he wears whenever someone points it out. He decided to ask Jim about it.
“In essence, Spock, to ride someone has a double meaning. The first to literally ride around on their back, the other is to have sex. Ride their dick, basically.” Jim didn’t seem to know if he was enjoying the explanation or embarrassed by it.
“I see,” Spock said slowly, responding to Jim’s expectant gaze. “Why does McCoy wear it?”
Jim grinned proudly “He’s about as close to a cowboy as you can get these days.”
(gets kinda nsfw under the cut, nothing explicit)