What was said? “That slide is very ‘inside baseball’ and i’m not sure we’ll present it.”
Did someone really say that? Yes, when reviewing a client presentation and seeing what slides we could cut/move to the appendix.
What does it mean? It basically means we are showing too much detail, that we are “in the weeds” (another idiom!).
Origin: The term dates back to the 1890s and was used to talk about… you guessed it, baseball! It referred to an offensive strategy of playing baseball, developed by the Baltimore Orioles and focused on keeping the ball in the infield via singles, walks, bunts and stolen bases (sometimes it was called Baltimore baseball or Oriole baseball).
Within a few decades, the phrase “inside baseball” came to signify technical details & highly specialized knowledge of playing the game, details that would only be appreciated by fans and “insiders,” and therefore irrelevant to outsiders. By the 1950s it was being applied to politics using the concept that you didn’t need ALL the electoral votes, only 270 of them – highlighting strategies for candidates to not spread themselves too thin.
In the example provided above, the slide that was shown had too many nuanced details/minutiae that required specific knowledge to be unintelligible/irrelevant to someone unfamiliar with the topic.
HOT Idioms 
Good one!
I believe the Baltimore Orioles did not originate until the 1950s.