To look for a needle in a pottle of hay.

It describes a search so difficult that success is unlikely: the desired thing is tiny, while the material through which one must hunt is dense, plentiful, and unhelpful.

The saying can also criticize an inefficient task or express frustration at having to locate one crucial detail among many irrelevant items. “Pottle,” now unfamiliar to many readers, evokes a quantity of hay and sharpens the contrast with the tiny needle.

Use it when emphasizing the near-impossibility of finding a particular object, fact, or clue amid a large, disordered mass. It is especially apt for searches whose effort seems out of proportion to the chance of success; readers may need context for the word “pottle.”