needle in a haystack
An item that is very hard or impossible to locate, as in Looking for that screw in Dean's workshop amounts to looking for a needle in a haystack. Originating in the early 1500s, with meadow instead of haystack, this metaphor exists in many other languages as well.
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The American Heritage® Idioms Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.