Green’s Dictionary of Slang

an intensified form of SE unbelievable; thus unfuckingbelievably adv.

1967–8		Baker et al. CUSS 217: Unfuckingbelievable Extraordinary, unusual, hard to believe.
1973		R. Daley Target Blue 383: [T]he witness cried out, ‘Un-fucking-believable. That’s him. I’d know that voice anywhere’.
1976		(con. 1969) C.R. Anderson Grunts 62: Real unfucking believably smart!
1997		C. Hiaasen Lucky You 228: Chub said, ‘Un-fucking-believable.’.
2001		N. Barlay Hooky Gear 20: Unbefuckinlievable. She fuckin know the routine.
2001		‘Randy Everhard’ Tattoo of a Naked Lady 29: Un-fucking-believable!
2002		Simon & Burns ‘Old Cases’ Wire ser. 1 ep. 4 [TV script] Unbelievable. Unbefuckinglievable.
2006		B. Matthews Intractable [ebook] Crow picked up the joint a [...] ‘Unfuckingbelievable! A man has to come to jail to get a smoke’.
2018		‘Aidan Truhen’ Price You Pay 268: I tell you that was unbefuckinglievable. That guy – that was some shot man.