Speculation and Apex Artillery: All of This Fun

How does Luna not know what fun is?
This is what a lot of us asked when they first watched Luna Eclipsed (S02E04) and Luna asked Applejack " Fun? What is this fun thou speakest of?"
There was quite a bit of discussion about it, and the question was asked again when Princess Twilight Sparkle (S04E02) revealed that Luna, the Princess Who Did Not Know Fun, had held the Element of Laughter way back when.

Here's my take on this.

The perhaps simplest and also least likely explanation is probably the one that Luna didn't know what fun was because she never had any fun in her life before this point. There are three obvious problems with this explanation:

First, it makes no sense for a pony who represents the Element of Laughter to not know what fun or having fun is.
Second, not having had any fun ever would not be sufficient for Luna to not know fun. She would also have needed to never observe anypony else having fun, which seems absurd given that she watches over everypony's dreams and that she in fact turned into Nightmare Moon because she was jealous of Celestia because ponies had a lot of fun in the day but not so much at night.
Third, if Luna did know not fun, why was Nightmare Moon laughing so much?

Clearly, Luna must have known at least the concept of having fun and enjoying oneself, everything else would be too much of a stretch.

Another theory as to why Luna didn't seem to know what fun is concerns the evolution of language. Luna had not been around ponies for a millennium, so language was bound to be different, as evidenced by the fact that Luna spoke in the Traditional Royal Canterlot Voice and used the Royal We as thee and thou, all things not used in modern pony speech.
In fact, the English language itself did not have the word 'fun' a thousand years ago, so it is entirely possible that something similar is the case with the ponies' language.

There is a problem with this theory, even though it seems to explain everything so nicely.
If language had changed so much, how come that this, and just this word was the only one Luna had trouble understanding?
The closest analogue to the way Luna was speaking would probably be Early Modern English, which is only about four to five hundred years old, and not the Late Old English of a thousand years ago, which might indicate that language hadn't changed quite as much as to warrant Luna not knowing words.
The biggest problem presents itself in Princess Twilight Sparkle (S04E02), even more than the thousand years ago when Luna was banished, back when she and Celestia first used the Elements of Harmony and Discord told them he was having "so much fun."
So we know that Luna had known that word before.

Which leads me to my own personal explanation.
As I just mentioned, the word 'fun' was in use in Equestria more than a thousand years ago, in an era where everypony lived "in an eternal state of unrest and unhappiness." (S02E02) The word was in use by Discord the one who brought misery to the ponies and turned their lives upside down, the only one who was having fun while logic surrendered and chaos reigned.

What if its use by Discord shaped the ponies' understanding of the word 'fun' for the coming centuries?
What if a traumatised populace could only associate the word with the chicanery and shenanigans of the mad Spirit of Chaos?
What if it only gradually shifted to its modern, light-hearted meaning from a connotation of mischief, torment and upending of reality?

If that was the case, then Luna wouldn't be asking Applejack was fun was, she would be questioning the use of the word in the context of a celebration. In a way, what Applejack showed her, silly games that involved tossing pumpkins and plushy spiders, was in line with her understanding: nonsense for enjoyment's sake. This might even explain why Luna's reaction to the ponies being again afraid of her and not having "enough fun" was to animate the plushy spiders into creepy crawlies, almost something Discord might have done.

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I originally wanted this to go out yesterday, as a little Winter Solstice Special. Well, better late than never.
Have a nice day, everyone!