Green’s Dictionary of Slang

an onomat. term indicating one object hitting another with a dull thump, or a solid object falling into liquid; also as n. and v.

1884		E. Nye Baled Hay 91: The Cold, Damp Sea or the Murmuring Wave and its sad kerplunk.
1890–1904		Farmer & Henley Sl. and Its Analogues IV 93/2: Kerplunk.
1905		Marvel III:58 31: The billionaire [...] ejaculated ‘kerplunk!’ and, tripping over Bob’s lance, he dived head-first into his carriage.
1912		T.A. Dorgan Silk Hat Harry’s Divorce Suit 31 Oct. [synd. cartoon strip] Bang! Zowie! Ka-plunk! The three men rushed in.
1914		R. Lardner ‘Horseshoes’ in Coll. Short Stories (1941) 263: But I’ll give you a thousand bucks if the ball didn’t go kerplunk right into the third bag and stop.
1921		T. Thursday ‘Hail the Professor’ in Top-Notch 1 Sept. 🌐 The rock come down—kerplunk, right on his head.
1929		(con. 1917–18) C. MacArthur War Bugs 112: Another gun-carriage going kerplunk into the soup.
1936		Puffy 12 Feb. [synd. cartoon] ‘Roar’ goes the engine and Puff falls ker-plunk.
1939		W. Winchell On Broadway 4 Jan. [synd. col.] Maurice Chevalier has fallen with a ka-plunk for a songstress in Paree.
1958		Casey ‘Kid’ Motsisi ‘Beedee Stuff’ Casey and Co. (1978) 25: One of them kaplonks me on the head.
1963		New Yorker 29 June 26: That’s why I nearly went kerplunk when you walked out of here with him .
1980		in S. Terkel Amer. Dreams (1982) 24: Kerplunk went her American Dream.
1996		Penguin Bk of More Aus. Jokes 112: He gets to the crest of the bridge across the island when, snap, bang, kerplunk, the car goes bung .
2000		R. Antoni Grandmother’s Erotic Folktales 192: A heavy metal object came ka-plonk! down on top of my head.
2000		F. Kellerman Stalker (2001) 427: Rina let her keys fall from her hand until they dropped to the ground; the kerplunk was audible.
2006		P. Shannon Davey Darling 72: I hit the dirt, whack, and lifted my head to hear the bolts bounce off the tubes kerplunk plang plang.