Green’s Dictionary of Slang

an intensified form of SE guarantee.

1970		P. Roth My Life as a Man (1974) 19: I guaranfuckingtee you gentlemen, not one swingin’ dick will be leavin’ this fiddlefuckin’ area to so much as chew on a nanny goat’s tittie.
1977		L. Heinemann Close Quarters (1987) 17: Light up, it won’t kill ya. ’Bout the only thing round here that won’t. Guaran-fucken-tee!
1990		S. Morgan Homeboy 21: Rooski [...] guaranfuckinteed the chink pharmacist went home at six.
2000		G.V. Higgins At End of Day (2001) 28: If that’s where they’re comin’ from, Nova Scotia’s where the crew’s gonna end up — guaranfuckintee it.
2001		‘Randy Everhard’ Tattoo of a Naked Lady 75: You’ll think twice before opening your big mouth again, ah guaran-damn-tee.