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China is a rich country, yet food prices are sky high.

The word 'sky' seems an adjective.

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2 Answers 2

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It's an idiomatic way of saying extremely high, exorbitantly high. Sky is not an adjective here; much rather, both words form a single entity that can work as an adjective or an adverb. You will often see it hyphenated, sky-high.

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There are many more-or-less idiomatic expressions which qualify an adjective by a noun expressing a conventional standard. They're particularly common for colours "rose red", "snow white", "sea green", "sky blue"; but also found for other qualities: "sky high", "lightning quick".

For colours they seem to be fairly open-ended - you can certainly replace the standard, as in "cherry red", "blood red", "pillar-box red". But I think the non-colour phrases are more conventional: "mountain high" or "skyscraper high" would sound odd.

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If you are Ike & Tina Turner, or one of their admirers, "river deep, mountain high" might not sound odd. –  RedGrittyBrick Feb 28 '11 at 15:50
@RedGrittyBrick: if that was meant seriously, I think it's a different case, poetic for "deep river, high mountain". If you think about it, while "mountain" is a plausible standard for "high", "river" is an unlikely one for "deep". –  Colin Fine Feb 28 '11 at 18:38

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