E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The tradition is that when our Lord was on His way to Calvary, a robin picked a thorn out of His crown, and the blood which issued from the wound falling on the bird dyed its breast with red. (See CHRISTIAN TRADITIONS.)
Robin Redbreasts. Bow Street runners were so called from their red waistcoats.