Literally, it means "not more further", i.e. it is the peak, there is nothing better, this is the ultimate thing compared to others (like, you could say "my Mod

Literally, it means "not more further", i.e. it is the peak, there is nothing better, this is the ultimate thing compared to others (like, you could say "my Model X is the non plus ultra of electric SUVs", just for example). I guess it is not a common expression but I like the phrase. ;-)