Is he yelling "kusnav" or like "kusnov"?
What does it even mean ... couldn't find anything on the webs - am I heavily mistyping it?
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The only similar-sounding word that I recall hearing in Yuri!!! on Ice is the Russian word vkusno (вкусно), which in the show is sometimes used by characters to mean "tasty". (For instance, it appears in ep. 2 when Victor is introduced to katsudon, and when in ep. 9, Yuri Katsuki tastes the katsudon pirozhki that Yuri Plisetsky's grandfather made.)
The IPA transcription of вкусно is [ˈfkusnə]: it is then not implausible that a casual listener might miss the initial [f] sound or be confused when transcribing the final schwa, leading to a transcription like kusna or kusno. (When encountering unexpected two consonant combinations, I have sometimes missed the first consonant entirely: for instance, when I first heard the Hebrew song "Hatikvah", I heard "tsofiyah" (צופיה) as "sofiyah," because it was easier for me to process what I heard that way when I was not listening carefully.)