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.
Entries linking to splosh
splash v.
1715 (intransitive); 1722 (transitive), probably an alteration of plash with an intensive s-. Related: Splashed; splashing. Splash-board attested from 1826. Splash-down (n.) in the spacecraft sense is attested from 1961.