That one is clear to me, although it might be for a reason you find displeasing: if the thing being computed was an "inverse square root" in the sense of a functional inverse, then you'd be computing a square and it would make much more sense to call it that instead.
It sounds like an algorithm for computing the inverse of the square root function, so one might think it's for squaring non-negative numbers, or something along those lines. Not so. It's for computing the reciprocal (the 'multiplicative inverse') of the square root of a number. [0]
Related to this: the way a superscript '2' next to a function means the function shall be applied twice (that is, composition with itself)... unless it's a trigonometric function, in which case it means the square of the result. [1] [2]
[0] https://en.wikipedia.org/wiki/Fast_inverse_square_root
[1] https://en.wikipedia.org/wiki/Function_composition#Functiona...
[2] https://en.wikipedia.org/wiki/List_of_trigonometric_identiti...