No "Fun": Noun? Yes. Adjective? Well...

The journalism professor was not having much "fun" explaining things to her feature-writing students: "I know so fun is wrong but I can't tell them why," she wrote. "So happy is right, but so fun should have 'much' as the sandwich filling."

If you ask practically anybody under 35 whether "so fun" is acceptable English, you will probably be told "Duh," or today's equivalent of eye-rolling disbelief.

But if you ask usage authorities, you'll get differing opinions. And, as is so frequently the case, the answer on whether "so fun" is "correct" will depend on the publication, the audience, and the context.

The "fun" in "so fun" ("That ride looks so fun") is acting as an adjective, the way "big" is an adjective in "That ride looks so big." Traditionally, however, "fun" is only a noun. That's why adding the sandwich filler "much," an adjective, to render the expression as "so much fun" makes it seem proper.

For a long time, though, people have been saying "I had a fun time," using "fun" as an adjective. After all, as Garner's Modern American Usage points out, we have adjectives for nearly all of our other emotions: "exciting," "fearful," "gloomy," "sad." Even so, Garner's calls "fun" as an adjective a "casualism," and lists it only at Stage 3 of the Language-Change Index, meaning many educated people use it, but would be smart to avoid it in a college entrance essay.

The Oxford English Dictionary says "fun" began as a verb in English in 1685, possibly derived from a variant of a verb "to befool." It was followed 14 years later by the first use of "fun" as a noun, the OED says. Showing its British stiff upper lip, the OED does not even list an adjectival form of "fun" except as parts of other adjectives, as in "fun-loving."

Other authorities have a little more fun. A usage note in The American Heritage Dictionary (fifth edition) says that "fun" as an adjective "has become widespread and must be considered standard, though writers may want to avoid it in more formal contexts."

The dictionary used by both The New York Times and the Associated Press, Webster's New World College Dictionary (fourth edition), says "fun" as an adjective is "informal." But how that's applied differs between those institutions. The New York Times Manual of Style and Usage is in the OED's corner: "Though the commercials may someday win respectability for fun as an adjective (a fun vacation), the gushing sound argues for keeping the word a noun."

The Associated Press Stylebook is silent, though the "Ask the Editor" archive weighs in against "funner" as "slangy."

Nearly everyone, in fact, opposes "funner "and "funnest" as anything but kid-speak or deliberate irony. ("Funniest," of course, is okay, but it doesn't mean the same thing as what's intended by "funner "or" funnest.")

If you want to use "fun" as an adjective, consider your audience: Would the English teachers (or journalism professors) reading it leave scathing comments? If so, leave it out. Or would avoiding it make you sound like a drip? Then go ahead and have some fun.