Wiktionary:Votes/pl-2013-09/CFI and trimming the Idiomaticity section

Compounds are generally idiomatic, even when the meaning can be clearly expressed in terms of the parts. The reason is that the parts often have several possible senses, but the compound is often restricted to only some combinations of them.

For example, mega- can denote either a million (or 2²⁰) of something or simply a very large or prominent instance of something. Similarly star might mean a celestial object or a celebrity. But megastar means "a very prominent celebrity", not "a million celebrities" or "a million celestial objects", and only rarely "a very large celestial object" (capitalized, it is also a brand name in amateur astronomy).

This rule must be applied carefully and is somewhat subjective. For example, bank has several senses and parking lot has an idiomatic sense of "large traffic jam". However bank parking lot can’t possibly mean "to put a large traffic jam in a financial institution". With such clearly wrong interpretations weeded out, the remaining choices are "place to park cars for any of several kinds of business"” or "place to park cars by, for or on a river bank or similar" (as opposed to, say, the hill parking lot). The whole phrase could plausibly mean either, depending on context (though the first is likely far more common), and so the phrase is not idiomatic.