E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A dude (q.v.); an exquisite; a lardy-dardy swell who dresses æsthetically, behaves killingly, and thinks himself a Romeo. This sort of thing used to be called crushing or killing, and, as mashing is crushing, the synonym was substituted about 1880. A lady-killer, a crusher, a masher, all mean the same thing.
The prattle of the masher between the acts, Daily Telegraph, Oct. 10, 1883.