Whisky Lord-Lieutenant whisky. Mr. Kinahan being requested to preserve a certain cask of whisky highly approved of by his Excellency the Duke of Richmond, marked it with the initials L.L., and ever after called this particular quality L.L. whisky. The Duke of Richmond was Lord-Lieutenant from 1807 to 1813.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894