E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
In the battle of Bannockburn (1314) King Robert Bruce ordered all the servants, drivers of carts, and camp followers to go behind a height. When the battle seemed to favour the Scotch, these servants, or gillies, desirous of sharing in the plunder, rushed from their concealment with such arms as they could lay hands on; and the English, thinking them to be a new army, fled in panic. The height in honour was ever after called The Gillies Hill. (Sir Walter Scott: Tales of a Grandfather, x.)