Green’s Dictionary of Slang

the slapping or splashing noise of something that hits a hard surface.

1897		‘H.S. Merriman’ In Kedar’s Tents 55: A bullet went ‘splat’ against a rock near at hand, making a frayed blue mark upon the grey stone .
1982		Beano 26 June 6: Splat!
1999		Guardian Rev. 10 July 10: Dropping an egg – splat – on a bald man’s head.
2000		Guardian Rev. 14 Jan. 5: People’s brains going splat! against the windshield.