Ichthyomancy - Divination by the heads or the entrails of fishes
JRS - Joe Robbie Stadium, the Marlins home field
The A-Train - the D-Train, Dontrelle Willis
Skump - very similar to slump
Send him to the hotel - Seal the deal
Cy Cobb - Dontrelle Willis
Bumnnies - no one knows really. You'd have to ask Wiggins
GRD - conventionally referred to as a ground-rule double, which is really a "rule book" double - if, you know, you want to be technically correct
Oogie - Danny, Dan Uggla
Although it probably makes no sense, entries are listed in the order in which they were added. I'm sorry that they're not alphabetical, or something else logical.
Feel free to add any that are missing in the comments...