The noise that something squashy makes when it hits a hard surface. In UK comics, when this happens, you will usually see the word in capitals coming out of the interaction between hard surface and the squashy material. Excellent example of onomatopoeia.
Suitable materials for making the SPALT sound are things like tomatoes, mashed potato, custard pies and squelchy mud.
Look in UK comics like the Beano and Dandy for examples.
The custard pie hit the boys face:- SPLAT!