"Heck" is to "Hell" like "Darn" is to "Damn" (or "Freaking" to “F--ing") - a word that sounds similar but is more polite, to be said in public in more religious places and times.
It's a form of hella which is "hell of" but hecka sounds silly so you get heckin. (Heck is also used as a substitute for fuck sometimes and takes its forms.)
This still doesn't make sense as "hellin" isnt a term in wide use. What makes the most sense is transition from "fuckin". But why not simply use fuckin? Surely invoking sex is less taboo than invoking hell.
