If a hiss is like a hisss than what is a hissing hiss?

A "hissing hiss" means a louder, sharper or more pronounced "hiss."

Explanation:

First, let's check out the meaning of the word "hiss."

Hiss, when used as a noun, is a sound that is like the letter "s." So, the first part of the sentence shows the similarity of the word "hiss" to the sound when pronouncing the word "hisss."

On the contrary, a "hissing hiss" shows the relationship of the adjective "hissing" to the noun "hiss." "Hissing" is an adjective here which describes the kind of "hiss." It gives more information about it. So, a hissing hiss means a louder, sharper or more pronounced "hiss."

showing or having an insensitive and cruel disregard for others. (so basically being heartless)

hope this : )