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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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up vote 1 down vote accepted

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

Actually, all above answers are wrong. Sorry. The term was first used to describe the price of land increases on the Isle of Skye in Scotland after the English crown forcibly removed the highlanders and paid landowners to raise sheep. Since it became more profitable to raise sheep than house workers, land prices rose enormously in a short period of time. Thus forced many to leave the island for somewhere more affordable.

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Can you provide references for this assertion? If not, your answer is apt to be downvoted and may even be deleted. – tchrist Aug 18 '15 at 0:20
It's not a good an idea to be dictatorial about etymology. There are so many ways that new things enter a language. I think you must provide evidence for your view. Here's some which indicates that your claim is wrong. Google Ngram ... – Margana Aug 18 '15 at 9:24
...The Ngram shows a first use of "sky high" in 1797. This predates the Skye clearances by over forty years. – Margana Aug 18 '15 at 9:29
Also, there is no indication in "Skye's Hidden Heritage- the Clearances" that the English Crown was instrumental in the clearances, and "Wikipedia" claims that English employees involved in the process were working for "hereditary Higland chiefs". – Margana Aug 18 '15 at 9:48
This is really interesting, but it's probably better as a comment unless you can provide a reference. – Dog Lover Aug 18 '15 at 11:01

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