The story
From splat + book, from the tendency for all the books in the series to be named uniformly, with only the name of the group varying. See splat (“asterisk”), a symbol used as a wildcard in computing.
Etymology from Wiktionary, CC BY-SA 4.0.
splatbook
Today it means: One of a series of books for a roleplaying game, oriented towards players, each of which covers a specific group with further options for player characters in that group. Full definition →