The expression mummy porn is new in 2012, its coinage directly linked to the massive popularity of the E. L. James novel. The use of mummy is presumably inspired by the fact that this type of novel appeals mostly to middle-aged rather than younger women, women who are therefore more likely to be ‘mothers’. Porn is of course short for the word pornography (images intended to give sexual excitement), and the expression’s success lies partly in the shock factor associated with this word, which usually has rather negative, risqué overtones.