It means rain is on the way. This saying seems to be more widespread around the country nowadays, but many - including Eric Partridge in his �Dictionary of Catch Phrases' - believe it started life in the East Midlands. The best explanation I've ever heard as to its origin is the one that suggests the �Bill', �Will' or �William' in the phrase refers to William (Will/Bill) Shakespeare. He was born in Stratford-on-Avon, to the west of the Midlands and - if the sky is dark and threatening over that area (ie over Bill's mother's) - it means rain is probably headed for the east of the Midlands region, because the prevailing winds are likely to push the wet weather in that direction.