Что такое Дзаккэнаё?

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It is a slang version of this expression:

ふざけるなよ！(fuzakeruna yo!)

It means, "Stop messing around!" (or, Stop fooling around, or something like that)

"ふざける" means to: act funny, act up like a child, fool around, kid around, mess around, screw around, etc. (you should get the point now)

"な" is the negation of the verb in command form.

"よ" is a particle usually used to end an informal statement.

* Tip: Say "fuzakeruna yo" fast for more than 20x and you'll hear yourself saying "zakennayo".

Cм. здесь:

http://answers.yahoo.com/question/index?qid=20080820054613AAhZYz7