Before I answer your question, I would like to correct a common misconception that is included in your question: slang words are not actual words.
Slang words are words that belong to the language from which they originate. I think the question you mean to ask is about the register of the word “ginormous”. Register is a term that deals with how formal or informal a word or phrase is or in which situations words/phrases can/cannot be used. Slang words in English are on this register scale just as much as academic English is; they have different registers because they are used in different situations.
Slang terms are considered to be among the most informal registers. They are generally used in casual conversation with people you know well, especially when the slang is still relatively new, or in very specific places (such as on the Internet). Slang terms may surpass these uses or these places and be used in other places. An example is the word “cool” meaning “fashionably attractive or impressive”. This was originally a word that was used in the world of Jazz in the 1920s. This word has the potential to be used nowadays in many different situations and by many different groups of people; it is no longer true “slang”.
To answer your question (as others have mentioned before), ginormous has been used since the Second World War (definitely from 1948, maybe also from 1942) and is a combination of the words “gigantic” and “enormous”. While some dictionaries still state that this word is slang, I would disagree. Dictionaries are slow to change to reflect the way words are used. I would consider it to be informal. I don’t use “ginormous” very often, but if I needed to impress upon someone that something was not just “gigantic” or “enormous”, I may use “ginormous” even with someone I don’t know well or in formal contexts (e.g. at work with my boss). However the word (maybe it is just my opinion) does have a somewhat childish connotation (as in I think a child is much more likely to say it than an adult) as things are larger to children.