To remain on guard, to watch out for trouble. In maritime language keeping a weather eye means looking toward the wind to observe weather conditions and look for squalls. The term appeared in a Sailor’s Word-Book (Smyth, 1867) but by the end of the nineteenth century was being used figuratively for keeping any kind of careful watch. Thus Lee Thayer used it (Murder Is Out, 1942), “You know how to keep your weather eye lifting.”