From "hell of (a)", like hella (originally Northern California slang). See also Wikipedia:Hella (word).
Meanings
(Northern-California, not-comparable, slang) Intensifier (modifies verbs); to a large extent; totally; very much.
“[…]. This helluv hurts.” We grownups burst out laughing, and I hugged her bony adolescent shoulders. 2002, Peggy Vincent, Baby Catcher: Chronicles of a Modern Midwife, page 225:
Dude, I helluv want to go to the show with you, but I'm soooo grounded.
(Northern-California, not-comparable, slang) Intensifier (modifies adjectives); to a large degree; extremely; exceedingly.
“We had the baby, like two minutes ago. It was helluv icy. […] Naeema was so brave, just hellabitchin', and I delivered the baby myself 'cause we have this hellafresh midwife who taught me how to do it.” 2002, Peggy Vincent, Baby Catcher: Chronicles of a Modern Midwife, page 226:
That new girl is helluv sexy.
“Man, if I had never been to California and read that magazine, I would be helluv scared to come out there!” 2003, Adrienne Anderson, Word: Rap, Politics and Feminism, page 14:
Shortened form, or elision, of phrases like "hell of" or "hell have", approximating casual speech in writing.
Meanings
(colloquial, contraction) Elision of "hell of".
“That's the helluv it — beggin' yer pardon, miss...” 1997, Kat Martin, Innocence Undone, page 297:
Oh, you goddam-betcha, Julie, it's one helluva lot better — one HELLUV A lot better — to be born dirt-poor. Course then you get to stay that way the rest of your life. 1977, Curt Johnson, The Morning Light, page 214:
(colloquial, contraction) Elision of "the hell of".
“Helluv it is,” he grumbled. “I was thinking of marrying her myself. Wrote her a letter, but it didn't get posted in time.” 1997, Kat Martin, Innocence Undone, page 173:
(colloquial, contraction) Elision of "hell have"
Where the helluv you been? 1973, Richard Cowper, Clone, page 99: