"Reverse rape" is a sometimes-used alternate term for female-on-male rape, but I couldn't begin to tell you how the term was formulated to refer to that specifically.
If I could guess, I think it refers to role reversal, with the (rational, going off of statistics) assumption that rape is primarily an act perpetrated by a man, against a woman.
the use of "reverse" is just a word used to distinguish on how inverse a trope goes. Usually when a trope is mentioned e.g. traps, immediate association of it as "guy who looks/dress/acts too good and is more femininely attractive than females". We had, for years, held Hideyoshi as the prime example of traps to the point where it was jokingly made that Hideyoshi itself is the gender.
So if we use "Reverse" and add it to the trope we get Reverse Trap and the prime example of it is Subaru from Mayo Chiki.
The trope can be applied both ways but adding "Reverse" to it specifies on what sub-type of the trope it was. So in this instance, "Reverse Rape" specifies female-on-male action. Rape is still rape, nothing is different but since the word itself is so attached and stigmatized to "male-on-female", the use of "reverse" just makes it clear that what happened was the other way around. Who knows how this usage of words for tropes will last, we have an entire light spectrum of genders now so phreak it... ¯\
(ツ)/¯
Hope my explanation was understandable. I feel like my brain struggle sometimes with what I write nor recognize what I've written kek.
I'm getting too old now.
