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
落进干草垛里的一根针(比喻如大海捞针般难找的东西)
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.
英汉词典释义
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.你连这句话在书的哪部份都不知道,我们怎么找得到呢?这简直是大海捞针。