A dude (q.v.); an exquisite; a lardy-dardy swell who dresses aesthetically, 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 aots.” Daily Telegraph, Oct. 10, 1883.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894