E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The initial letters of Grand Old Man; so Mr. Gladstone was called during his premiership 18811885. Lord Rosebery first used the expression 26th April, 1882, and the Right Hon. Sir William Harcourt repeated it, 18th October, the same year; since then it has become quite a synonym for the proper name.