a combination of mini and microscopic. in other words, it means very very small.
you scratched my car you fucking asshole!
what, you mean that miniscopic little speck? i can barely see the damn thing!!
Small, to the point of near invisibility; The opposite of "ginormous."(Comes from "miniscule" and "microscopic")
"She told me last night that my chances of getting her phone number were miniscopic."