A haystack is a large, solid pile of hay, often covered with a straw roof to protect it, which is left in the field until it is needed.
PHRASE
落進干草垛里的一根針（比喻如大海撈針般難找的東西）
If you are trying to find something and say that it is like looking for a needle in a haystack, you mean that you are very unlikely indeed to find it.
英漢詞典釋義
n.
干草堆
How can we ever find the quotation if you don’t even know what part of the book it comes from?It’ll be like looking for a needle in a haystack.你連這句話在書的哪部份都不知道,我們怎么找得到呢?這簡直是大海撈針。