"excessive or monotonous use of distinctive methods in art or literature," 1803, from manner + -ism. Meaning "an instance of mannerism, habitual peculiarity in deportment, speech, or execution" is from 1819. Related: Mannerisms.
Perhaps few of those who write much escape from the temptation to trade on tricks of which they have learnt the effectiveness; & it is true that it is a delicate matter to discern where a peculiarity ceases to be an element in the individuality that readers associate pleasantly with the writer they like, & becomes a recurrent & looked-for & dreaded irritation. But at least it is well for every writer to realize that, for his as for other people's mannerisms, there is a point at which that transformation does take place. [Fowler]