Ζουλάπι isn't a Greek word. It means "wolf" or "fox" and it's used to describe people that are insidious like those animals but instead of directly calling the person a "fox", you call him "ζουλάπι" to a little disrespect for him too. So I choose to translate that as "insidious fool" although as with all slang words it doesn't necessarily have an aggressive tone. The text could be talking about a friend of the author that wished him yesterday (for his birthday or something) and the author teasingly calls him "ζουλάπι".