I am constantly told "funner" is not a word. Even Google auto corrects. Yet "funner" is used very often in spoken English with people I meet.
Is funner a word? If not why?
What causes it to not be a word?
The AHDEL has this USAGE NOTE:
The use of fun as an attributive adjective, as in a fun time, a fun place, probably originated in a playful reanalysis of the use of the word in sentences such as It is fun to ski, where fun has the syntactic function of adjectives such as amusing or enjoyable. The usage became popular in the 1950s and 1960s, though there is some evidence to suggest that it has 19th-century antecedents, but it can still raise eyebrows among traditionalists. The day may come when this usage is entirely unremarkable, but writers may want to avoid it in more formal contexts.
So fun can hardly be called a central adjective even though it is here conceded to have an existence as an adjective. Thus, arguments that 'funner' and 'funnest' must therefore be acceptable cannot be taken as read - 'merest' exists, but not 'merer'.
The best one can say is that some people accept funner and funnest as allowable words and others don't, that both schools have pretty good arguments on their side, and that the supporters of the usages will almost certainly come out on top in a few years.
Yet "funner" is used very often in spoken English with people I meet.
This means that it is a word. Just like "homie" is a word.
Words don't have to be used often to be words. For instance, "cryptosporidium" is a word which is used less often by the people you meet than "funner".
Yet, the anchorwoman of your six o'clock news show would probably use "cryptosporidium" when appropriate, while avoiding "funner" in favor of "more fun".
For some reason, "funner" is a disfavored in what my linguistics professor used to call the "socio-economically preferred dialect", which is basically the educationally corrected language used by the socio-economic elite of society, at least in formal communication.
This is a far cry from "funner" actually not being a word.
If you intend to make a speech in the preferred dialect, then use of the word "funner" is probably inconsistent with your objective.
It is curious why, because "fun" is in a similar lexical category as, say, "green", and yet "greener" is not disfavored in the same way. The possible reason is that "fun" wasn't always in this category; so to speak, it is a late-comer into this category of words. Once upon a time, it was not possible to use "fun" as an adjective. Perhaps more time is needed for "fun" to be more fully integrated into the preferred dialect as a fully fledged adjective. Meanwhile, people not sticking to that dialect, of course, just use the rules of the language as they are intended. There is no such thing as not a fully fledged dialect in natural language. Once we consider "fun" to be an adjective, we have a legitimate basis for using "funner" and "funnest". If we do not believe so, then it means we do not consider "fun" to be an adjective and should also avoid saying things like "I had a fun time" rather than "I had fun".
If an English language learner uttered the words
This is a funner film
I'm pretty sure that native speakers would either correct him, "This film is more fun" or skip past the error but immediately sense that something was off. However, in the many years of teaching English privately to students I can say, hand on heart, I've never come across this type of language mistake by Italian learners (I cannot vouch for other nationalities), on the contrary they often produce the following
This is the more funny film
The holiday was funny
The first is the result of an L1 (first language) interference while the second is grammatically correct and in the right context be very acceptable, but in most cases the ESL students meant to say, fun. Why bother correcting these errors in speech and writing if "more" and "funny" are words that exist and can be found in any dictionary? Because, as I explain to students, English native speakers do not normally say these sentences and communication might break down.
Language learners tend to be a highly motivated lot and want to (some desperately need to) communicate effectively in English. Prescriptive grammar is a necessary evil in order to help non-native speakers sound more natural. For the very same reasons, funner and funnest may be classed as words, found in dictionaries and may even be uttered by native speakers, it does not mean phrases or sentences that contain these expressions are standard... not yet.
Type "funner" in Google, and the first page will show people asking whether funner is a word or not. On that same page is the Grammarist's viewpoint which I'll quote because it argues in favour for "funner" quite convincingly
The reason the use of funner and funnest has been discouraged is that fun was until recently only a noun. Nouns do not have comparative (-er) and superlative (-est) forms, but mass nouns such as fun can be modified by more and most (e.g., “I have more water,” or “he has the most courage”). But while some of the stodgier English reference books still pretend fun is not an adjective, most English speakers moved on long ago, and the adjectival fun is rarely questioned. Ultimately, if we accept that fun is an adjective—and we have no choice, because it’s common—then we also have to accept funner and funnest. Comparatives and superlatives of one-syllable adjectives usually take the -er and -est endings, and there’s no good reason fun should be any different.
A well-argued case for funner and funnest being the logical and natural comparative and superlative forms for fun. Yet, I doubt I could ever bring myself to say one day, let alone write,
“This holiday was funner”.
“That was the funnest holiday”.
On the other hand, Urban Dictionary1 which purportedly reports on the most up-to-date language developments has this to say about funner
The dumb person's way of saying 'more fun'
I suspect the majority of English speaking people (whom neither ELU, nor the Grammarist represent) think this on hearing and reading "funner".
"Funner" is not a word, because "fun" is a noun, not an adjective. You can't turn a noun into a comparative, unless it has an adjectival form, which this does not.
Otherwise you'd be making comparisons between different "clocks", for example, by saying that clock "A" was clocker than clock "B".
People say a lot of things that aren't proper grammatical English. So what? People exceed the speed limit and rob banks, too. Doesn't make it right. Although it might be more funner to race a car than drive like an old lady. Or old man.
As @Whitecat points out, I am wrong to say that "fun" is not used as an adjective, because it is, sometimes, used as an adjective. Nevertheless, it is not an adjective, as the Google Ngram in @Jeremy's answer suggests.