A cloudy speck which indicates the approach of a storm. When Elijah heard that a speck no bigger than a “man's hand” might be seen in the sky, he told Ahab that a torrent of rain would overtake him before he could reach home (1 Kings xvii. 44, 45). Thomson alludes to this storm signal in his Summer.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894