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1.
A Japanese curse word which comes from:

Fuzakeru ふざける (巫山戯る): to romp; to gambol; to frolic; to joke; to make fun of; to flirt; to mess around; to screw around; to josh; (taken from EDICT)

Fuzakenaide ふざけないで is the negative request form of the verb, asking that the listener to desist from messing around. Sticking ~yo on the end gives it emphasis.

'zakenna is a slangy drawled version of the above. To sound tougher and scarier add ~zo instead of ~yo, though you might need a shiny suit and a tight perm to pull it off.

NB: This is considered very rude in all its forms and, though it sounds fairly innocuous when translated literally into English, would be taken as a 'Fuck you!" or "Stop fucking about!" by a native.
zakennayo

もうやめなさい。今日はやけにふざけているんだから。
mou yame nasai. kyou ha yakeni fuzakete iru'n da kara

Quit already! 'cos today you're just pissing about.
by mudskipper April 22, 2011
 
2.
A rarely used way of saying "Fuck You" in Japanese.
Jimmy: Nice ass.

Yunko: Zakennayo, loser.
by Ukchana December 24, 2003