The crest of this family is a winged spur. or spur between two wings, leathered, with the motto. “Nunquam non paratus.” When King Edward I. was meditating treachery in favour of Balliol, Johnstone sent to Bruce (then in England) a spur with a feather tied to it. Bruce took the hint and fled, and when he became king conferred the crest on the Johnstone family.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894