Green’s Dictionary of Slang

echoic of the sound of an object hitting the ground.

1916		P. MacGill The Great Push 57: ‘We’ll ’ave a drink to ’elp us,’ said Bill, and a cork went plonk!
1920		Marvel 14 Aug. 5: It lifted him clean off his feet, and laid him down with a loud plonk on the floor.
1934		Kent & Sussex Courier 13 Apr. 4/4: As he [i.e. a bus driver] was crossing the cross road there was a ‘plonk,’ and he pulled up.
1941		G. Kersh They Die with Their Boots Clean 2: [He] goes down plonkety-plonk on the road.