Citation, Documentation of Sources

Q. Dear Manual of Style: My friend and I are having a disagreement about whether or not “smoking gun” must be hyphenated when used as an adjective (i.e., smoking-gun evidence vs. smoking gun evidence). He believes that it is appropriate to hyphenate, citing CMOS. I believe that when the hyphen is unnecessary to help a reader differentiate a compound adjective from two adjacent adjectives that each independently modify the noun, it is unnecessary to hyphenate (e.g., chocolate chip cookie, high school teacher). Which one of us is correct?

A. I agree with you, but someone has to decide whether the adjectives are safe without the hyphen. If you want to be absolutely sure that no one will think the teacher is high on something or the evidence has been set on fire, you should add a hyphen to “high school” and “smoking gun.”