how do you pronounce the word niesamowity and what does it mean?
Actually the answer is not as simple, as you were led to believe, because - irrespective of the gender - the adjectives niesamowity, niesamowita and niesamowite may have several meanings: the original one, and then the transformed one. [The word "niewiarygodny", which you are tracking in another thread, follows the similar process.]
Since the adjective "nie-samowity" starts with the negation "nie" the question is: is there a word "samowity"? Well, not in the current use, but it existed in Old Polish and its etymology is somewhat related to the word "sam" and the verbs "widzieć" or "wiedzieć", which are - by the way - also related, not only in Slavic languages. With rough approximation the word "samowity" derives from "samo-wit' ", "samo-widzieć", "samemu widzieć". I would not be surprised if the modern fashionable "samowiedza" had the same root. In any event "Opowieści samowite" by Wojciech Kuszczok might be worth to read in order to explore the concept.
The last dictionary of Polish language, which still carried the definition of adjective "samowity", was "Słownik języka polskiego" PWN 1958-1969, edited by W. Doroszewski. "Samowity" was defined as "naturalny, przyrodzony" - natural, innate.
So, in opposition to that the adjective "niesamowity" would mean "weird, unnatural, eery, uncanny". And that's the basic meaning in all three genders: masculine, feminine, neuter.
It might be worth pointing out that German language has similar pair of adjectives, although their meanings do not map exactly to Polish, which is due to different historical culture involving vampires and all that stuff:
heimliche => tajemny, skryty, sekretny, tajny, ukryty
unheimliche => niesamowity, przedziwny, tajemniczy, straszny, złowieszczy
As I already pointed out, meaning of the words change. As the English word "terrific" has transformed from "frightening, fearsome, horrible" to become "great, wonderful and splendid" so the Polish "niesamowity" became "amazing".