1889 [in Farmer, who calls it "A New England variant of splash"], ultimately imitative. Perhaps influenced by splish-splosh "sound made by feet walking through wet" (1881). Related: Sploshed; sploshing.
We had to splosh across the wet meadow
splinter
split
split-level
split-screen
split-second
splosh
splotch
splurge
splutter
Spock
spode