Zakennayo is short for the phrase "Fuzakeru na" which literally means "Don't joke around." It might also be heard as "fuzakennayo" or "fuzakenna." Depending on the context, you would translate it as "don't fuck with me" or (in a non-literal strecth) "cut the shit" or a simple "don't mess with me," "quit joking around," or "You've got to be kidding me!" This term is usually used by punks or yakuza trying to look tough, but is used by many other people as well. It was also the subtitle of a daytime drama series that aired in Japan called "Kid's War ~Zakennayo~", and was often used as the catch phrase of the show.