Green’s Dictionary of Slang

(US) wonderful or very bad.

1971–73		G. Sheehy Hustling 50: Legs is what a man looks for...and you got one hellifying pair of legs [HDAS].
1986		D. Barker (con. 1925) Life in Jazz 80: She made the sign of the cross for my safe return from the ‘hellfying state of Miss’sippi’.
2001		Indianapolis Star 7 Feb. 🌐 E01: J.J. can play some hellifying stuff but makes it look easy.