a partially submerged alluvial fan that has merged with a delta.
Baseball Slang True Fans Should KnowThe game dates back a long way (the first pro team started in 1869—the Cincinnati Reds) so there’s a long history here.
Origin of fan delta
First recorded in 1915–20
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019