Helluva is a slang contraction of hell + of + a. This is an expression (in the non-contracted form) that dates back to the early peasants of the British Empire. Back then, it was assumed that anything extremely bad or undesired came from hell, because that was what Christianity taught them. Therefore, they said "a hell of a bad storm" to mean "the worst storm hell had to impart on them". (Of course, helluva does not only apply to storms) This was almost always an exaggeration used by the peasants more often than the royalty.
The use of the word "hell" in "-hell of a-" is not only incorrect, but interesting as to why. Hell is usually the demarcation for a place in the afterlife for sinners and evildoers (according to the bible), yet, in the expression, it is used as an adjective. Hell is not an adjective. Other than here, it is rarely used as one. So why is Hell used like that? Simple- peasants had horrendous grammar. Since most of them were very uneducated, they created and/or modified words to express their ideas when they could not think of one.
"Helluva", as well as "Hell of a", are still considered inapropriate phrases in polite speech. Even then, Hell was considered profanity, and so its use was avoided except at the most necessary of times. Religion taught the people that the mentioning of Hell's name would cause them great problems, and so they used it sparingly. Since hell is profane (for most formal terms), the whole phrase is considered rude and degrading.
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