The tutelary saint of the house of Douglas is St. Bridget. According to tradition, a Scottish king in 770, whose ranks had been broken by the fierce onset of the Lord of the Isles saw, the tide of battle turned in his favour by an unknown chief. After the battle the king asked who was the “Du-glass” chieftain, his deliverer, and received for answer Sholto Du-glass (Behold the dark-grey man you inquired for). The king then rewarded him with the Clydesdale valley for his services.
“ `Let him not cross or thwart me.' said the page; `for I will not yield him an inch of way, had he in his body the soul of every Douglas that has lived since the time of the Dark Gray Man.' ” —Scott: The Abbot, chap. xxviii.
The Good Sir James, the dreadful blacke Douglas' That in his dayes so wise and worthie was, Wha here and on the infidels of Spain, Such honour, praise, and triumphs did obtain.
The person generally called “Black Douglas” is William Douglas, lord of Nithsdale, who died in 1390. It was of this Douglas that Sir W. Scott said -
“The name of this indefatigable chief has become so formidable, that women used, in the northern counties, to still their froward children by threatening them with the Black Douglas.” —History of Scotland, chap. xi.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894