E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
When the good Lord James, on his way to the Holy Land with the heart of King Robert Bruce, was slain in Spain fighting against the Moors, Sir Simon Locard, of Lee, was commissioned to carry back to Scotland the heart, which was interred in Melrose Abbey. In consequence thereof he changed his name to Lock-heart, and adopted the device of a heart within a fetterlock, with this motto: Corda serrata pando (Locked hearts I open). Of course, this is romance. Lockhart is Teutonic, Strong Beguiler.
For this reason men changed Sir Simons name from Lockhard to Lockheart, and all who are descended from Sir Simon are called Lockhart to his day.Sir Walter Scott: Tales of a Grandfather, xi.