Guaran-damn-tee or Guaran-dam-tee (a firm guarantee)

A “guaran-damn-tee” (or “guaran-dam-tee”) is the strongest possible guarantee, with “damn” added for emphasis. “I’ll guarandamtee ya” has been cited in print since 1950, when Richard Pike Bissell wrote about the Mississippi River. “Not the Texas legislature, I guarandamtee” was spoken by a character in the novel The Searchers (1954), by Alan Le May, about the Texas-Indian wars (later made into a 1956 John Ford western film starring John Wayne). “Guaran-damn-tee” has been popularly used in Texas, but the term does not appear to have originated in any particular state or from any particular source.
 
“Guaran-damn-tee” has been frequently used in professional wrestling since at least the 1990s.
 
   
Wikipedia: Tmesis 
Tmesis ( /ˈmiːsɨs/ or /təˈmiːsɨs/; Ancient Greek: τμῆσις tmēsis, “a cutting” < τέμνω

temnō, “I cut”) is a linguistic phenomenon in which a word or set phrase is separated into two parts, with other words occurring between them.
(...)
“Guaran-damn-tee” in which an expletive or profanity is inserted for humor and/or emphasis.
 
Urban Dictionary
guarandamntee
the ultimate guarantee
vince mcmahon - i guarandamntee i will kick your ass
by Matthew Fisher Dec 3, 2003
   
Google Books
A Stretch on the River
By Richard Pike Bissell
New York, NY: New American Library
1951, ©1950
Pg. ?:
‘Well, one thing, I says, I’ll guarandamtee ya I ain’t agoing to wheel no mo’ coal until we pumps out that fuel flat and real good.”
     
Google Books
The Searchers
By Alan Le May
New York, NY: Harper
1954
Pg. 98:
“Not the Texas legislature, I guarandamtee.”
 
Google News Archive
21 June 1955, Vancouver (BC) Sun, “From Our Tower” by Dick Beddoes, pg. 13, col. 1:
VANCOUVER SUN, Oct. 29, 1953.—“Mr. Stukus is a very oft-spoken person. On the subject of Vancouver as a football centre, he spoke: ‘Enthusiasm, heck! This town’s loaded with it, I’ll guaran-damn-tee you.’...Well, if the town isn’t, I’ll guaran-damn-tee you Mr. Stukus is.”
   
Google Books
Violent Saturday
By W. L. Heath
New York, NY: Bantam Books
1956, ©1955
Pg. 55:
“I said forget the car. I’ll handle it, and I guaran-dam- tee you it’ll go off like silk.”
 
Google News Archive
29 May 1960, The News and Courier (Charleston, SC), “‘Progress’ Soon To Hit The Bayous” by Hugh A. Mulligan (AP), pg. 3C, col. 4:
“Eh bien, it ain’t much fun being married twice as old as youself to a woman. I guaran-dam-tee you dat.”
     
Google Books
Esquire
Volume 61
1964
Pg. 403:
There were two boys from Alabama who believed in segregation and would guaran-damn-tee ya’ that Governor Wallace would either be president or kingmaker in ‘64, but on the other hand they were as friendly toward the Negroes as anyone else.”
 
28 May 1964, Portsmouth (NH) Herald, “A ‘Rump-Sprung Old Dog Soldier’” by Richard Starnes, pg. 4, col. 6:
“I don’t know when it was I had my last serious disciplinary problem and I will guaran-damn-tee you there isn’t a more combat-ready outfit on earth today.”
(From “old Col. McNoguff, as we’d best call him”—ed.)
     
Google Books
The Summer Land
By Burke Davis
New York, NY: Random House
1965
Pg. 53:
“But I’m aiming to double my money on her. I’ll guarandamntee you that.”
 
Google Books
Moonshine Light, Moonshine Bright, a novel
By William Price Fox
Philadelphia, PA: Lippincott
1967
Pg. 338:
“Coley, dammit, this is an automatic. It won’t take more than a couple hours. We just start her up, mix in the water, and then carry it out to O’Hara’s. He’ll buy every drop with no questions asked. I can guaran-damn-tee that.”
 
12 August 1970, The Evening Times (Trenton, NJ), “Town Can’t Forget Murderous Spree; Starkweather’s Girlfriend, Now 26, A Bitter Reminder Behind Bars” by Tom Tiede (NEA), pg. 25, col. 8:
“But if you lived here, and knew those good innocent Lincoln (Nebraska—ed.) folks who died, then I guaran-damn-tee you that you’d hate the girl, too, and you’d want to see them keep her locked up for the rest of her life.”
 
Google Books
The Old Man and Lesser Mortals
By Larry L. King
New York, NY: Viking Press
1974
Pg. 241:
“Well if he ever does”— the cowboy paused for effect and waved his beer mug— “I bet and guaran-damn-tee you it’ll have electric lights, velvet fairways, and dancing girls for caddies.”
   
Google News Archive
2 March 1975, The Blade (Toledo, OH), sec. G, pg. 11, col. 3:
Language Gets Crackin’
From Crackers Expert
IF NOTHIN’ DON’T HAPPEN. By David M. Newell. Knopf. 242 Pages. $7.95.
By LOU KLEWER
Georgia Crackers aren’t the only Crackers. There’re a lot of them in Florida, too, with their own regional dialects, their own particularly peculiar way of talking, of mixing verbs and pronouns, and even their own way of inserting a four-letter cuss word into another word, just to guarandamntee that it will be a mite stronger.
 
11 August 1977, The Times-Picayune (New Orleans, LA), “Moonshiners Are Growing Marijuana” by James Branscome (Washington Post), sec. 2, pg. 5, col. 4:
“If a man can have a couple of cigarettes in his shirt pocket without fear of arrest, I guaran-damn-tee you it’s going to get real popular,” predicted North Carolina ATF agent Westra.
     
Neoseeker Forums
I Guaran DAMN Tee it
deadman walking
Oct 03, 03 at 9:27pm
I got good news about the little battle between RAW 2 and HCTP.