Green’s Dictionary of Slang

(US) an onomat. term indicating the sound of two solid objects colliding; also adv. see cite 1875.

1836		M. Clarke Poems of M’Donald Clarke 223: His cradle was the leeward bunk, / Where he bawled the Storms a clumsy chorus, / And often tumbled out, cochunk, / As the whirling waves came slapping o’er us.
1845		Ottawa Free Trader (IL) 30 May 1/4: Just at that moment, somethin’ gin way above, and may I die ef Betsy [...] didn’t drope rite through the floor [...] cochunk flat into the pan of mush!
1847		Chunkey’s Fight 128: Co chunk ! went Jem into the middle of the floor.
1850		Odd Leaves 50: We reached the oaks, Colt tried to pass Preacher, Preacher tried to pass Colt, and cowollop, crosh, cochunk ! we all come down like persimmons arter frost.
1875		Iola Register (KS) 30 Jan. 1/5: Well hoss we expect you to be right cochunk up to the hub on them questions.