E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Never turn a loaf in the presence of a Menteith. Sir John Stewart de Menteith was the person who betrayed Sir William Wallace to King Edward. His signal was, when he turned a loaf set on the table, the guests were to rush upon the patriot, and secure him. (Sir Walter Scott: Tales of a Grandfather, vii.)