I am trying to find a book series from the 80-90s whose main character talked about practicing "yuwinitsu" or you win when it suits you. It was post nuclear with a war between humans and mutant slug like creatures. I just can't remember the name. Any help appreciated
According to this random link ( https://mcsmith187.wordpress.com/tag/eagleheart/ ) it is Eagleheart Book 1: Silver Wings and Leather Jackets, by C. T. Westcott, the first book in a trilogy. That link includes a description of someone
who teaches [the main character] the art of Ywon-itsu the code of the American samurai. Translated it means you win when it suits you.
Also the link mentions the RIHMs (Radioactive Induced Human Mutations) which could be the slug-like creatures you mentioned (although they are not described further).
The Amazon reviews mention that the book is set "in post apocalyptic America" but nothing else that matches your description. The Amazon page also says the book's copyright is 1989, which fits your timeline.