large, white cumulonimbus. The large, white snowy-looking clouds that rise along the horizon. . . were called Banff bailies, and at all seasons of the year were looked upon as the forerunners of foul weather (Walter Gregor Notes on the Folk-Lore of the North-East of Scotland 1881).
A bailie was originally the baron’s deputy in a burgh of barony and it was later used, and in some places still is used, of certain civic dignitaries. So these clouds are, depending on the bailie, quite imposing and stately or a bit puffed up.