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.
1736, "water or liquid thrown upon anything," from splash (v.). Meaning "striking or ostentatious display" is first attested 1804. Sense of "small quantity of soda water, etc., added to a drink" is from 1922. Of color or light, 1832.