Green’s Dictionary of Slang

(US) of falling, heavily or noisily, with a thump.

1848		‘Dow, Jr.’ Short Patent Sermons i 166: Ere the middle round [of the ladder] be reached, the bottom slips, and down you come ker-flummux.
1855		Spirit of Times (N.Y.) 29 Sept. 387/1: And the fust thing you knows he falls and down he comes kerflumix .
1908		DN III 326: He fell kerflummox off the bench.
1911		N.Y. Tribune 15 Jan. 35/1: When they get heart failure [...] they go down kerflummox.
1919		E. Dyson ‘Bricks’ ‘Hello, Soldier!’ 30: At Bullcoor mortal charnce had dumped a mutton-truck of us / From good ole Port ker-flummox where we didn’t orter be.