Green’s Dictionary of Slang

1. (US) to confound, to flabbergast, also used reflexively, to become confused, etc; also as n., high-flown speech, kerflummoxed, confused, ‘in a state’.

1850		Manchester Spy (NH) 21 Sept. n.p.: ‘An’ if they wasn’t off I’ll co-flumux ’.
1867		in Iowa Journal of Hist. LVII (1959) 227: Teetotally ‘kerflumixed’ with emotion [HDAS].
1870		Eve. Teleg. (Philadelphia, PA) 22 Jan. 7/1: Too much high-sound talk [...] is called ‘highfalutin’ and sometimes ‘keflummux’.
1882		Nat. Police Gaz. (NY) 18 Feb. 11/3: Colored first society in Santa Fe, N. M., is all ‘kerflummuxod’ .
1883		G.W. Peck Peck’s Bad Boy and His Pa (1887) 96: It kerflummoxed Ma when I went into the dining room [,...] and broke Pa all up.
1889		Dead Bird (Sydney) 17 Aug. 1/2: A William-street butcher was recently kerflumaxed by a nice litle thing in petticoats who gravely requested him [etc] .
1890		E. Field ‘The Conversazzhyony’ Little Bk of Western Verse 157: But I wuz kuflummoxed when Hoover said he’d choose.
1901		Boys of the Empire 23 Apr. 37: Hed ter make up, hyer an’ now, er be etarnally kerflummaxed.
1905		L. Pound ‘Dialect Speech in Nebraska’ in DN III:i 62: kerflummux, v. Bewilder, daze. ‘I felt quite kerflummuxed.’.
1911		‘Roy Rockwood’ Lost on Moon 141: I knowed dat no old meteor could kerflummox us!
1916		El Paso Herald (TX) 18 July 7/1: He will be so kerslostrated, not to say kerflummoxed, that he won’t know what to do.

2. to ponder.

1897

Kipling Captains Courageous 65: When dad keflummoxes that way [...] he ’s doin’ some high-line thinkin’ fer all hands.

3. (US) to fall heavily or noisily.

1877		E.L. Wheeler Deadwood Dick in Beadle’s Half Dime Library I:1 81/3: When a feller kerflummuxes rite down onter a payin’ streek I opine he’s goin’ ter roost thar till get gits reddy to vamoose.
1884		E. Nye Baled Hay 223: The sudden kerflummix of one of these cheerful articles of furniture.
1901		T. Hammond On Board a Whaler 44: ‘Somebuddy doughter to tell ’im,’ chimed in another. ‘Ye-ah, an’ git kerflummoxt all over the deck fer ’is pains,’ cautioned a third.
1903		Paducah Sun (KY) 3 Jan. 4/4: My mother would exert a sudden pressure on my shoulder and down I’d kerflummox in that icy water.
1906		J.W. Carr ‘Words from Northwest Arkansas’ in DN III:ii 143: kerflummox, v. To fall down in a heap. ‘He kerflummoxed on the ice.’.
1909		J. Ware Passing Eng. of the Victorian Era 162/1: ‘Ker’ is also frequently used before words implying movement, as kersmash, kerbang, kerash (crash), kerflummux, kerslap.