Is he yelling "kusnav" or like "kusnov"?
What does it even mean ... couldn't find anything on the webs - am I heavily mistyping it?
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.)
Viktor Nikiforov yells the word 'Vkusno' (вкусно), which is the Russian word for 'delicious'. If you notice, he only yells it when eating Pork Cutlet Bowls (not a euphemism) and Hot Pot!!