Green’s Dictionary of Slang

(US) to hit hard.

1954		J.D. MacDonald All These Condemned (2001) 135: There was one particularly nasty little scene when Wilma whonked Randy with a mallet.
1970		L. McMurtry Moving On 307: Woody whonked into the Coke box, almost rupturing himself.
2001		ucferrarisgirl The Assault on Monica at www.fanfiction.net 🌐 A feather-covered Jake laughed as he whonked Frank on his backside with the pillow.
2008		Crab Apple Cove 18 Jan. [blog] You [...] grab up the old tomcat by the tail, and then you whonk his nuts with this here bald-peen hammer.