Q: My husband and I have always said things like “That was such fun” or “This is so much fun.” Nowadays, I hear “That was so fun.” Is this new usage acceptable?
A: No, the usage isn’t acceptable, but it’s now so common that someday it just might be.
“Fun” is traditionally a noun (a thing, as in “We had fun”), not an adjective. So you usually wouldn’t use it as a modifier (“We had a fun day”). An exception would be when “fun” is a predicate nominative – a noun that follows a verb and modifies the subject (“This is fun”).
Therefore, it would be OK to use “fun” in a sentence like “Skiing is fun,” but not in one like “We had a fun day on the slopes.”
For the same reason, the ubiquitous and annoying “so fun” is incorrect, but “so much fun” is not. If you mentally substitute a noun like “entertainment” you can see why. You wouldn’t say “so entertainment,” but you could say “so much entertainment.” Similarly you could say “This is entertainment” (predicate nominative).
I can’t say exactly when “so fun” came into being. (I don’t see any published references for the expression in the Oxford English Dictionary.) But I think it was probably inevitable.
People are used to putting “so” before predicate adjectives – that is, adjectives that describe the subject of a sentence (“This is so pink” or “This is so hard”). It’s natural, if not proper, that some people would want to put “so” before a noun that describes a subject (“This is so fun”). And if enough people do it, “fun” may become a legitimate adjective one day.
One more thing: Although “fun” is primarily a noun, it has been used since the 17th century as a verb meaning to make fun or to cheat (this usage is now obsolete), according to the OED.
Now, isn’t English fun!
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