A Zepper is someone here who favors Ukraine in the Ukraine/Russia war. It’s a take-off on Zelenskyy, Ukraine’s president. Just as “Putinista” is someone who favors Russia.
Both are meant to be insults, of course.
(And insults are never helpful in conversations - just my opinion.)
Frequently, insults are used by people who know they're wrong to cover up the fact that they're wrong. Even more frequently, insults are used by enemies of free and intelligent discussion to suppress free and intelligent discussion.