why you hyphenate phrasal adjectives

I just left a comment over at John McCrarey's blog, which says in part:

Trump and Sharpton ought to settle their current differences by having some sort of weird-hair contest.

Why hyphenate "weird hair"? Because it's a phrasal (or compound) adjective, and the hyphen indicates that the two words together form a single concept. Were I to leave the phrase unhyphenated, you wouldn't know whether it's the hair that's weird or the contest that's weird: is it a (weird hair) contest or a weird (hair contest)?

Rule of thumb: hyphenate all phrasal adjectives that precede the nouns they modify.
     a once-in-a-lifetime opportunity
     the now-former Miss Michigan
     a dead-on impersonation
     some history-making events
     his once-loyal centipede
     my dog's Jesus-shaped asshole

Is there an exception to this rule? Arguably, yes: if the phrasal adjective, as is, is completely unambiguous, then a hyphen isn't needed.

The classic example given as an exception is the phrase "high school," as in "a high school band." Hyphenating "high school" might come off as stilted and pretentious, although in all honesty, I'd hyphenate the phrase, anyway, out of an old-school sense of obligation. But many grammar references allow for the "unambiguity" exception, so I'll grudgingly admit that "high school band" is kosher as is, although I personally still read the phrase and hear echoes of a marching band of teen potheads. To me, then, there's at least a whiff of ambiguity.

One famous example of when you must hyphenate is:

The meteorologists attended a violent-weather seminar.

Imagine what happens when the hyphen drops off.

So there you have it: as a rule of thumb, hyphenate phrasal adjectives that precede the nouns they modify, unless you determine the phrasal adjective to be utterly unambiguous.

Oh, yes—one more caveat: if the first word of your phrasal adjective is an adverb ending in -ly, do NOT hyphenate:
     a quickly wilting erection
     her slowly dawning horror
     a seriously ugly scrotal wart
     some strangely alluring lepers

And while we're at it, keep in mind that many words ending in -ly might look like adverbs, but they're actually adjectives, to wit: friendly, kindly, kingly, cowardly, beastly, treacly, bodily, ghostly, godly, squiggly, heavenly, otherworldly, ugly, etc.

Trivia: friendly can legitimately be made into the adverb friendlily.