Why not?
Well, "fun" is not an adjective; it's a noun, like "brilliance", "hideousness" or "excitement" are nouns, but definitely not adjectives. A noun is that which is, not that which describes what is.
Yet, sadly, these are the mistakes people make all the time.
Imagine saying you had an "brilliance day", or you bought a "hideousness hand- cream", or you met an "excitement guy"? Saying that would be unimaginable, unless you were an inchoate second-language student.
Or worse still, bragging you got some "air fresh", blushing you had a "conversation lovely", or boasting you lead a "health life"? No, you'd never do that, unless you were an inchoate second-language student, or producing some pretentious French film, which, I suppose, amounts to the same thing in time.
Yes, as we all know, you can have a "fun-filled day", listen to a band, or buy a hand-cream, which engenders or evokes "fun", or meet a guy who is "fun-loving". Yes, you can, because you're not using "fun" to describe a noun; you're using it as a noun.
But you can't have fun both ways, I'm sorry. At least, you can't grammatically speaking, anyway.
Perhaps, owing to its status of prestigious desirability, people tend to thrash it about in their daily dialogues, as if using and misusing the noun as much as possible will enrich their lives with fun. Perhaps.
But it's just abysmally wrong.
"Yum" is worse. It is a meaningless, mindless word, and an abbreviated form of the slightly less nauseating "yummy". Yummy is a toddler's word, like "chockie bar"or "chippies", and that's fine for that demographic to use, but no one else. Yet people persist in distorting and contorting that word into the dreadful "yum", and end up using it 210 times a week to describe everything from beetroot to cronuts to soup to cereal.
Can I make a suggestion? Use another word, like scrumptious. It's not unlike yum in sound. Go ahead, use it. It's the lesser of two evils. Yum is dumb.
It sounds so insincere.
